The United States and Britain kave stepped up pressure on Russia to rein in Syrian President Bashar al-Assad, as Damascus's allies vowed to respond to any "aggression" against Syria.
U.S. Secretary of State Rex Tillerson said in an interview with ABC's This Week program aired on April 9 that in meetings next week in Moscow, he will call upon the Russian government to live up to a 2013 agreement that was intended to strip Syria of its chemical-weapons stockpiles.
"It agreed to be the guarantor of the elimination of the chemical weapons, and why Russia has not been able to achieve that is unclear to me," said Tillerson, who is to meet with Russian Foreign Minister Sergei Lavrov on April 12.
Tillerson’s comments come amid international outrage over an alleged chemical air strike on April 4 that Syrian activists say killed 86 people in the town of Khan Sheikhoun in Idlib Province.
Moscow and Damascus claim that the toxic gas was released when strikes by government forces hit a rebel weapons depot, an assertion the United States and others reject.
In response to the attack, the United States fired cruise missiles at a Syrian air base on April 7, drawing condemnations by Russia and Iran, which have given Assad crucial diplomatic and military support throughout Syria’s six-year-long civil war.
Washington's allies voiced support for the U.S. action, the first time the United States has intervened directly in Syria's conflict.
Tillerson made clear that the U.S. objective was limited to deterring further chemical attacks, saying, "Other than that, there is no change to our military posture."
The secretary of state stopped short of accusing the Russians of complicity in the attack, but said that "they've been incompetent and perhaps they've just simply been out-maneuvered by the Syrians."
If Syria carries out more chemical attacks, he warned, "that is going to be clearly very damaging to U.S.-Russian relations."
In an interview with CBS, Tillerson described Russia’s explanation for the suspected chemical attack as "not plausible" and "not credible."
He also reiterated that the top U.S. priority in Syria remained the defeat of the Islamic State extremist group, suggesting that Washington was not planning to actively seek Assad's ouster.
Meanwhile, a joint command center made up of the forces of Russia, Iran, and militias supporting Assad said the U.S. strike crossed "red lines."
"The aggression against Syria oversteps all red lines,” said a statement published by Al-Watan, a newspaper close to Assad's government. "We will react firmly to any aggression against Syria and to any infringement of red lines, whoever carries them out."
Meanwhile, Iranian Supreme Leader Ayatollah Ali Khamenei said the United States made a "strategic mistake" by attacking Syria, state news agency IRNA reported.
"The Islamic Republic of Iran will not leave the field...in the face of threats," Khamenei warned.
Also on April 9, Syrian state media reported that Iranian President Hassan Rohani affirmed his support for Syria's government in a phone call with Assad.
The Syrian leader was quoted as accusing the United States of trying to boost the morale of "terrorist groups" in Syria. Damascus refers to all Syrian opposition groups as "terrorists."
The Iranian president dismissed reports that Assad's forces were behind a chemical attack as "baseless," according to a statement on the presidency website.
"The nation of Iran will remain alongside the Syrian nation in fighting terrorism and safeguarding Syria's territorial integrity," he added.
The Kremlin said Russian President Vladimir Putin and Rohani condemned "aggressive" U.S. actions against Syria in a phone call.
Both presidents "noted the inadmissibility of U.S. aggressive actions against a sovereign state in violation of international law," an April 9 statement said.
It added that Putin and Rohani called for an "objective, impartial" investigation into the incident and expressed readiness to deepen cooperation to fight terrorism.
Russia also hit out at British Foreign Secretary Boris Johnson's decision to cancel a visit to Moscow, scheduled to take place on April 10, citing developments in Syria.
The move "once again confirms doubts about the added value of dialogue with the British, who don't have their own position on the majority of current issues,” the Russian Foreign Ministry said.
The British have "no real influence on the course of international affairs, remaining 'in the shadow' of their strategic partners," it added.
Johnson earlier said that instead of traveling to Moscow, he would now be going to Italy for a Group of Seven (G7) meeting on April 10-11 to build coordinated international support for a cease-fire in Syria.
He argued it would be best for Tillerson to deliver a "clear and coordinated message" to the Russians during his own visit to the Russian capital.
Johnson said in a statement that Britain deplored "Russia's continued defense of the Assad regime even after the chemical weapons attack on innocent civilians" and called on Russia to “do everything possible to bring about a political settlement in Syria.
Writing in the Sunday Times, British Defense Secretary Sir Michael Fallon blamed Russia for “every civilian death” in the “barbaric” chemical attack.
Fallon wrote that the Kremlin was responsible "by proxy" as the "principal backer" of Assad’s government.
"In the past few years, they have had every opportunity to pull levers and stop this civil war,” he wrote. “Russia must show the resolve necessary to bring this regime to heel."
With reporting by AFP, AP, Reuters, the BBC, TASS, and Interfax