US Ambassador to the United Nations Samantha Power says President Bashar al-Assad of Syria has barely dented his stockpile of chemical weapons.
"We assess that although Assad used more chemical weapons on August 21 than he had before, he has barely put a dent in his enormous stockpile," Ms Power told the Center for American Progress in Washington on Friday.
"We have exhausted the alternatives" to military action, she said.
"He must have weighed the military benefits of using this hideous weapon against the recognition that he could get away with it because Russia would have Syria's back in the Security Council," Ms Power said.
She said it was naive to think Russia would change.
Ms Power acknowledged that the US public is "ambivalent" about a military attack on Syria.
But she highlighted the precedent of Kosovo in 1999 for acting without UN Security Council approval and said it could help bring Mr Assad to the table to negotiate an end to the war.
"It is clear that Syria is one of those occasions, like Kosovo, where the (Security) Council is so paralysed that countries have to act outside it if they are to prevent the flouting of international laws and morals," Ms Power said.
Obama resists pressure to abandon air strikes
United States President Barack Obama has resisted pressure to abandon plans for air strikes against Syria and has won the support of 10 fellow leaders for a strong response to a chemical weapons attack.
Mr Obama refused to blink after Russian President Vladimir Putin led a campaign to talk him out of military intervention at a two-day summit of the Group of Twenty (G20) developed and developing economies in St. Petersburg.
Mr Obama has persuaded nine other G20 nations plus Spain to join the US in signing a statement calling for a strong international response.
However Reuters reports that's fallen short of support for military strikes, underscoring the deep disagreements that have dominated the summit.
Mr Obama has defended his call for a military response to what Washington says was a chemical weapons attack by forces loyal to Syrian ruler Bashar al-Assad that killed more than 1400 people in rebel-held suburbs of Damascus on 21 August.
The President is seeking Congressional support for a military strike against Syria.