Kerry: Syria chemical arms use 'obscenity'

UN chemical weapons experts meet residents at one of the sites of an alleged poison gas attack in the southwestern Damascus suburb of Mouadamiya August 26, 2013. [Photo/Agencies]

Assad has denied launching a chemical attack. The UN team came under sniper fire Monday as it traveled to the site of the Aug. 21 attack.

It's unclear whether Obama would seek authority from the UN or Congress before using force. The president has spoken frequently about his preference for taking military action only with international backing, but it is likely Russia and China would block US efforts to authorize action through the UN Security Council.

Kerry on Monday made several veiled warnings to Russia, which has propped up Assad's regime, blocked action against Syria at the UN, and disputed evidence of the government's chemical weapons use.

"Anyone who can claim that an attack of this staggering scale can be contrived or fabricated needs to check their conscience and their own moral compass," he said.

British Prime Minister David Cameron, who like Kerry cut short his vacation because of the attack, spoke Monday with Russian President Vladimir Putin to outline the evidence of chemical weapons use by Assad's regime.

Cameron's office also said the British government would decide on Tuesday whether the timetable for the international response means it will be necessary to recall lawmakers to Parliament before their scheduled return next week. That decision could offer the clearest indication of how quickly the US and allies plan to respond.

More than 100,000 people have died in clashes between forces loyal to Assad and rebels trying to oust him from power over the past two and a half years. While Obama has repeatedly called for Assad to leave power, he has resisted calls for a robust US intervention, and has largely limited American assistance to humanitarian aid. The president said last year that chemical weapons use would cross a "red line" and would likely change his calculus in deciding on a US response.

Last week's attack in the Damascus suburbs is a challenge to Obama's credibility. He took little action after Assad used chemical weapons on a small scale earlier this year and risks signaling to countries like Iran that his administration does not follow through on its warnings.