NATO Secretary-General Anders Fogh Rasmussen has called for a "firm" response to the use of chemical weapons in Syria.
At a news conference in Brussels on September 2, Rasmussen said he had seen evidence the Syrian regime was responsible for an August 21 chemical-weapons strike in a Damascus suburb.
"NATO's position is clear. The chemical-weapons attack that took place on August 21 around Damascus was appalling and inexcusable," Rasmussen said.
"We believe that these unspeakable actions which claimed the lives of hundreds of men, women, and children cannot be ignored."
Rasmussen said any military action "would be very short, sharp, tailored."
He added that it would be up to NATO's 28 members to decide what specific military response they might make.
The United States also blames the regime of Syrian President Bashar al-Assad for the attack, which Washington says killed more than 1,400 people.
The Syrian government denies the allegations and says the opposition used the chemical weapons.
On September 1, U.S. Secretary of State John Kerry said the United States had evidence that sarin gas was used in the attack. Kerry said the evidence had been presented to Moscow.
But in a speech to students at the Moscow State Institute for International Relations earlier on September 2, Russian Foreign Minister Sergei Lavrov said the evidence left "serious doubts."
"There are a lot of discrepancies, nonsense," he said. "Very much doubt."
"This is obviously a case of what we call 'double standards,'" Lavrov continued. "It is a clear case of personalities -- when a personal dislike toward an authoritarian leader leads to the idea that he must be overthrown by any means."
Lavrov added that he did not think the Syrian opposition will be more open to a political solution in the event of outside military intervention in Syria.
"If the action announced by the United States does take place -- to the great regret of all of us -- despite what they have said about Geneva-2 [the Syrian peace conference], the prospects of holding this forum will be put off for a long time, if not forever," he said.
The Russian foreign minister also urged the international community to immediately set a date for a conference on creating a zone in the Middle East that is free of weapons of mass destruction.
He accused the United States of blocking such a gathering and emphasized that Syria was one of the initiators of the proposal.
Kerry: Sarin Gas Used
U.S. President Barack Obama on August 31 said he was convinced Assad's government used chemical weapons in a Damascus suburb on August 21.
The Syrian government has denied carrying out any chemical attack, instead accusing rebel forces of responsibility.
Syria’s official "Al-Thawra" newspaper on September 1 ridiculed Obama, saying his move to seek the approval of lawmakers signaled the beginning of what it called a "historic American retreat."
WATCH: Syrians protest against possible U.S. strikes.
On September 1, U.S. Secretary of State Kerry said on the U.S. television program "Meet The Press" that the United States had evidence that sarin gas was used in the attack.
"Let me just add this morning a very important recent development, that in the last 24 hours we have learned through samples that were provided to the United States that have now been tested from first responders in east Damascus and hair samples and blood samples have tested positive for signatures of sarin," Kerry said.
Kerry expressed confidence that Congress would give its approval for the United States to launch strikes. He said the case for a military attack is "building and this case will build."
The Chinese Foreign Ministry on September 2 also said the U.S. evidence had been presented to Beijing but cautioned against "rushing to prejudge" the evidence before UN inspectors issue their report.
Obama is currently seeking U.S. congressional authorization for possible military strikes aimed at containing the Syrian government's ability to carry out chemical-weapons attacks.
Russian President Vladimir Putin on August 31 urged Washington to bring its claims to the United Nations.
The Syrian government denies the allegations and says the opposition used chemical weapons. Syria on September 2 sent a letter to UN Secretary-General Ban Ki-moon asking him to prevent "any aggression on Syria" and to continue pushing for a diplomatic solution to the conflict.
Arab League Urges Action
On September 1, a meeting of the Arab League foreign ministers adopted a resolution calling on the United Nations and the international community to take "deterrent and necessary measures" against those responsible for the use of chemical weapons in Syria.
The resolution also placed "full responsibility" for the August 21 incident on the Syrian government.
"Putting full responsibility for such a horrendous attack on the Syrian regime and demanding punishment and prosecution of all those involved in such a crime to be tried at international tribunals, as other war criminals have," said Arab League official Nassif Hitti, reading the statement for journalists in Cairo.
Earlier this year, the 22-nation Arab League gave Syria's seat in the bloc to the Syrian opposition coalition.
The statement came as talks among lawmakers intensified in Washington over Obama’s initiative to win congressional approval for a military operation against the Syrian regime.
Officials said Obama and some of his top aides called lawmakers to argue in favor of a strike, and national security officials held a private briefing. More talks are expected on September 2.
Reports suggest that many lawmakers remain unsure about how useful a limited strike, as Obama has called for, might be. Opinion polls say most Americans also are skeptical about what could be accomplished with a military operation and whether it would serve U.S. interests.
The Senate and House of Representatives are due to begin formally considering Obama’s request for authorization on September 9.
The United Nations said Secretary-General Ban had asked UN experts who collected samples from the site of the attack to conduct tests as soon as possible.
The UN said the samples would be sent on September 2 to laboratories around Europe, but it is not clear when the results will be available.
The UN chief has said no international military action targeting Syria should be taken before the inspectors have presented their findings. However, it is not part of the inspectors’ mandate to make a determination on who launched the attack.
Also on September 1, the Russian reconnaissance ship "Pryazovia" left Sevastopol, Ukraine, for the eastern Mediterranean on a mission to collect information about Syria, ITAR-TASS reported, citing a Russian Navy official.
With reporting by Interfax, ITAR-TASS, AP, AFP, and Reuters