US Secretary of State John Kerry is asserting the United States now has evidence of sarin gas use in Syria and says the "case is building" for a military attack.
A day after President Barack Obama stepped back from his threat to launch an attack, Mr Kerry said in a series of interviews that the administration learned of the sarin use through samples of hair and blood provided to Washington by first responders in Damascus.
Mr Kerry also said he is confident that Congress will give Obama its backing for a military strike against Syria. But the former senator also said the president has authority to act on his own if Congress doesn't give its approval.
Congress is scheduled to return from a summer break on September 9.
Mr Obama has unexpectedly stepped back from an imminent missile attack on Syria yesterday and instead asked the US Congress to support a strike punishing Bashar Assad's regime for the alleged use of chemical weapons.
The White House has sent congress a draft of a resolution authorising President Obama to use military force against Syria. It lays out the administration's claim that Mr Assad's regime killed more than 1,000 people in an August 21 chemical weapons attack.
It says the objective of a US military response would be to "deter, disrupt, prevent and degrade" the regime's ability to use chemical weapons and authorises Mr Obama to use the military as he determines "necessary and appropriate" to serve that goal.
The draft does not lay out a timeline for action, but does say only a political settlement can resolve the Syrian crisis.
With navy ships on standby in the Mediterranean ready to launch their cruise missiles, Mr Obama explained his decision to involve congress, saying he believed that as commander in chief, he had "the authority to carry out this military action without specific congressional authorisation".
But he added: "I know that the country will be stronger if we take this course and our actions will be even more effective." His remarks were televised live in the United States as well as on Syrian state television with translation.