Russia and the United States continued to trade barbs and accusations in the wake of a U.S. missile strike against a Syrian air base that Washington suspected was used by the Syrian military to launch a deadly chemical-weapons attack on civilians earlier this week.
In a post on social media on April 7, Russian Prime Minister Dmitry Medvedev said the U.S. move brought the country to "the brink of combat clashes with Russia."
Medvedev criticized U.S. President Donald Trump for acting "in defiance of international law, without approval from the United Nations, and in violation of [America's] own procedures that require Congress to be notified of a military operation."
He said Trump's decision to authorize the missile strike showed he was "dependent on the opinion of the Washington establishment, which the new president criticized so sharply in his inauguration speech."
Speaking at the United Nations, Russian deputy UN envoy Vladimir Safronkov said the missile strike was "a flagrant violation of international law."
He said Moscow was calling on Washington to "immediately stop the aggression and join the efforts on a political settlement in Syria."
Safronkov added that Moscow remains willing to work with the United States to combat terrorism, provided Washington stops dealing in "ultimatums."
Meanwhile, U.S. Ambassador to the United Nations Nikki Haley said that the U.S. missile strikes were "a very measured step" and warned that Washington is "prepared to do more -- but we hope that is not necessary."
Britain's UN ambassador, Matthew Rycroft, said Russia bears substantial responsibility for the apparent chemical-weapons attack.
"Russia has given [Syrian President Bashar al-]Assad everything he could dream of," Rycroft said. "Without Russia's seven vetoes in the Security Council, defying the views of other members of this council, Assad would have faced sanctions and justice."
In Moscow, Foreign Ministry spokeswoman Maria Zakharova said the planned April 12 visit to Russia of U.S. Secretary of State Rex Tillerson remains on the schedule.
"Let him come and tell what strange things they did," Zakharova said.
Earlier, Tillerson said Moscow deserved some blame for the apparent chemical attack because it has failed to live up to a 2013 agreement that was intended to strip Syria of its chemical-weapons stockpiles.
"Either Russia has been complicit or Russia has been simply incompetent in its ability to deliver on its end of the agreement," Tillerson said.
Nonetheless, Russia's Foreign Ministry also said Moscow was open to the possibility of resuming an agreement with the United States on preventing midair conflicts of Russian and U.S. aircraft over Syria. Earlier, Moscow said it suspended the agreement in the wake of the U.S. missile strike.
U.S. military officials, however, said the hotline between the two countries' militaries remains open despite Moscow's earlier statement.
Russian Defense Ministry spokesman Igor Konashenkov said the U.S. military attache in Moscow had been summoned and informed that the hotline would be cut off "as of April 8."
WATCH: Syrian Army Spokesman Says U.S. Strike Caused 'Significant Damage'
Speaking on condition of anonymity, a senior U.S. official told journalists that the United States "suspects" the Syrian military "had help" in carrying out the attack, but he did not explicitly accuse Moscow.
"At a minimum, the Russians failed to control the activities of their Syrian client," the official said.
U.S. military officials reported that a drone was observed over the area shortly before the chemical-weapons incident and then again afterwards, shortly before a hospital treating the wounded was also targeted by a Syrian air strike. The officials said they did not know whether the drone was Syrian or Russian.
The overnight missile strike was the first direct U.S. attack targeting Assad's government in six years of civil war and was Trump's most substantial action abroad since he took office in January.
The U.S. strike sharply increased tension between the United States and Russia, which immediately denounced the move as an "act of aggression" that would harm bilateral ties and serve the interests of terrorists.
Moscow said it would condemn the U.S. action at the next session of the cease-fire task force of the International Syria Support Group, which meets in Geneva.
The Syrian military said the U.S. strike killed six soldiers and caused extensive damage to the Shayrat Airfield in western Syria, which the Pentagon says was used to store chemical weapons.
Syrian state news agency SANA said nine civilians were killed in villages near the air base, including four children, and that seven others were wounded.
"What America did is nothing but reckless and irresponsible behavior, which only reveals its short-sightedness and political and military blindness to reality," Assad's office said in a statement.
Russia and Iran, Assad's chief allies in the Middle Eastern country's civil war, quickly condemned the strike in strong terms. Washington's NATO and regional allies voiced support for the U.S. action.
Russian President Vladimir Putin "considers the American strikes on Syria an aggression against a sovereign state in violation of...international law," Kremlin spokesman Dmitry Peskov said. He claimed the decision to strike was based on "an invented pretext."
WATCH: U.S. Warships Launch Cruise Missiles Against Syrian Air Base
"This step by Washington will deal a substantial blow to Russian-American relations, which are already in a lamentable state," and will "create serious obstacles to the creation of an international coalition for the fight" against terrorism, he said.
The Pentagon earlier said it told Russia in advance of the strikes and that it took pains to avoid hitting parts of the air base where Russian forces were stationed.
"Russian forces were notified in advance of the strike using the established deconfliction line," Pentagon spokesman Navy Captain Jeff Davis said, referring to a special military hotline.
Iran, which has given Assad's government major military and economic support, said the assault would "strengthen terrorists in Syria" and "complicate the situation in Syria and the region."
Iran condemns the use of chemical weapons but "believes it is dangerous, destructive, and [a] violation of international laws to use it as an excuse to take unilateral actions," the ISNA news agency quoted Foreign Ministry spokesman Bahram Qasemi as saying.
Support For Strike
U.S. allies threw their support behind the U.S. action.
German Chancellor Angela Merkel and French President Francois Hollande said in a joint statement that Assad bears "sole responsibility" for the U.S. action.
"Any use of chemical weapons is unacceptable, cannot go unanswered, and those responsible must be held accountable," NATO Secretary-General Jens Stoltenberg said. "The Syrian regime bears the full responsibility for this development."
WATCH: Australia Backs U.S. Strikes On Syria
European Council President Donald Tusk said the strike demonstrated "needed resolve" against "barbaric chemical attacks."
Israeli Prime Minister Benjamin Netanyahu tweeted: "In both word and action, [Trump] sent a strong and clear message today that the use and spread of chemical weapons will not be tolerated. Israel fully supports President Trump's decision."
Saudi Arabia gave "full backing" to the U.S. move, the official news agency SPA reported, citing an unnamed official at the Saudi Foreign Ministry.
Turkish Deputy Prime Minister Numan Kurtulmus also welcomed the strike, saying Assad's government should be punished and voicing hope "that this operation by the United States will contribute to peacemaking," state-run Anadolu news agency reported.
A spokesman for the British government said "the UK government fully supports the U.S. action, which we believe was an appropriate response to the barbaric chemical-weapons attack launched by the Syrian regime."
Japanese Prime Minister Shinzo Abe said his government "supports the U.S. government's determination to strongly oppose the spread and use of chemical weapons" and understood the strike as "an effort to prevent further deterioration of the situation."
'Crossed Many Lines For Me'
After what is believed to have been a nerve-gas attack on the Syrian town of Khan Sheikhoun on April 4 that killed more than 80 people, Trump said he was moved by photos of children being killed, which he said had "crossed many lines for me."
"Syrian dictator Bashar al-Assad launched a horrible chemical weapons attack on innocent civilians. Using a deadly nerve agent, Assad choked out the lives of helpless men, women and children," Trump said in a brief televised statement.
"It is in the vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons," which Assad used in violation of international law, he said.
"I call on all civilized nations to join us in seeking to end the slaughter and bloodshed in Syria," Trump said. "We hope that as long as America stands for justice, then peace and harmony will, in the end, prevail."
The U.S. military launched 59 Tomahawk missiles from two Navy destroyers in the Mediterranean Sea, the USS Porter and USS Ross, at about 0040 GMT on April 7, targeting the Shayrat base.
"These missiles targeted aircraft, hardened aircraft shelters, petroleum and logistical storage areas, ammunition supply bunkers, air defense systems, and radars," Pentagon spokesman Davis told the media
"Initial indications are that this strike has severely damaged or destroyed Syrian aircraft and support infrastructure and equipment at Shayrat Airfield, reducing the Syrian government's ability to deliver chemical weapons," Davis said.
The Britain-based Syrian Observatory for Human Rights said that the "air base was almost completely destroyed -- the runway, the fuel tanks, and the air defenses were all blown to pieces."
The Reuters news agency quoted a U.S. official who spoke anonymously as saying the cruise-missile was a "one-off," suggesting that no further strikes are currently planned.
Russia and Syria claim that the toxic gas was released when air strikes by government forces hit a rebel weapons depot, an assertion the United States and others reject.
Peskov, Putin's spokesman, asserted that "the strikes were de facto carried out in the interests of IS, the Al-Nusra Front, and other terrorist organizations."
"It can be said unequivocally that these strikes have damaged the cause of fighting terrorism," Peskov said ahead of a meeting of Putin's Security Council to discuss the situation.
Russia's Defense Ministry, meanwhile, said that Syrian air defenses -- which rely in part on Russian antimissile systems -- will be strengthened.
"To protect Syria's most sensitive infrastructure, a complex of measures will be implemented in the near future to strengthen and improve the effectiveness of the Syrian armed forces' air defense system," said ministry spokesman Konashenkov.
He also claimed that the U.S. strike's effectiveness was "extremely low," saying that "only 23 missiles reached the air base. It is unclear where the other 36 fell."
WATCH: Russia Slams 'Thoughtless' U.S. Strikes On Syria
Trump did not announce the attack in advance, though he and other national security officials issued warnings to the Syrian government throughout the day on April 6.
"I think what happened in Syria is one of the truly egregious crimes and shouldn't have happened and it shouldn't be allowed to happen," Trump told reporters traveling on Air Force One to Florida, where he was meeting with Chinese President Xi Jinping.
The air strikes won approval from some leaders of both parties in the U.S. Congress.
"It is critical that Assad knows he will no longer enjoy impunity for his horrific crimes against his own citizens, and this proportional step was appropriate," said Senate Foreign Relations Committee Chairman Bob Corker.
"Making sure Assad knows that when he commits such despicable atrocities he will pay a price is the right thing to do," said Senate Democratic leader Chuck Schumer.
But some U.S. lawmakers criticizing Trump for ordering the use force without seeking their approval.
"Congress will work with the president, but his failure to seek congressional approval is unlawful,” said Senator Tim Kaine, the 2016 Democratic vice presidential candidate.
With reporting by AP, AFP, Reuters, TASS, RIA Novosti, Interfax, and NBC News