U.S. Secretary of State John Kerry says independent testing has confirmed the use of sarin gas in last month's attack on civilians in the Syrian capital of Damascus.
Kerry told U.S. media Sunday that samples provided to the United States from first responders on the scene have "tested positive for signatures of sarin".
His comments come a day after U.S. President Barack Obama announced that he will seek congressional approval before launching a military strike against the Syrian government in retaliation for what the U.S. concluded was the use of chemical weapons.
Mr. Kerry says he is confident that members of the U.S. Congress "will do what is right", but adds that the president has the power to act no matter what Congress decides.
Syria's state new agency quotes President Bashar al-Assad saying Sunday that Syria was capable of confronting any "external aggression". The country's deputy foreign minister, Faisal Maqdad, said Mr. Obama's decision to seek approval from Congress was full of "hesitation and confusion".
The Syrian government alleges that the the August 21 attacks were carried out by rebel fighters, but has not yet presented any evidence supporting the claim.
Syria's opposition says it is disappointed with Mr. Obama's decision to hold off on taking quick action against Damascus. The opposition coalition said in a statement Sunday that any possible military action should be carried out in conjunction with the Free Syrian army.
In an address at the White House Saturday, Mr. Obama said he has decided the United States should take military action against Syrian government targets. But he said that while he believes he has the authority to order a strike, he also thinks it is important for the country to have a debate on the issue.
Later, President Obama formally asked Congress to allow him to use military force in Syria to "deter, disrupt, prevent and degrade" the potential for more chemical attacks. The president ruled out any action that would put American ground troops in Syria.
He called what happened in Damascus nearly two weeks ago the worst chemical weapons attack of the 21st century, and he said the U.S. must not turn a 'blind eye" to it.
U.S. congressional leaders responded by saying they expect the Senate and House of Representatives to take up the matter when they return from their summer recess the week of September 9.
A U.N. inspection team wrapped up its work in Syria and left the country Saturday. A U.N. spokesman says in a call with Secretary-General Ban Ki-moon, the head U.N. inspection team said the preparations for classifying the samples are progressing well and that they will begin to be transferred to laboratories on Monday.
The spokesman says Mr. Ban asked that the analysis of the samples be expedited and that a report of the results be sent to him as soon as possible. The secretary general also said that he believes the U.N. Security Council should stand "firm and united" in agreeing on measures in response to any use of chemical weapons.
The Syrian government has denied having any role in chemical weapons attacks. But Mr. Obama said U.S. intelligence is clear that "well over 1,000 people" were murdered - gassed to death by their own government.
In another development, French Interior Minister Manuel Valls is reported to have said Sunday that France will not take action alone against Syria, and will wait for a decision by U.S. Congress as Paris needs to be part of a coalition.
Protesters around the world took to the streets on Saturday to protest for and against a possible U.S.-led attack on Syria.
Amnesty International issued a statement calling on the U.N. Security Council to refer the Syrian situation to the International Criminal Court, to impose an arms embargo on the Syrian government, and to deploy international monitors to investigate and report on human rights abuses in Syria.