At the State Department on Friday, spokeswoman Marie Harf said that "verifying, accounting for securing and destroying a large stockpile of chemical weapons takes time," adding that "it's very difficult to do, particularly in an active war zone."
"If we keep forward momentum, if we believe there's a credible and verifiable plan on the table to do just that, we'll keep moving forward with that process, because resolving this issue diplomatically is certainly preferable to resolving it or to dealing with it with military action," Harf said.
Obama had tried to put together a NATO coalition to attack Syria, but the British Parliament voted against taking part, denying him a normally reliable ally. Other allies said they wanted U.N. authorization in the form of a Security Council resolution before they would join a coalition.
The president then asked Congress to authorize a military response in Syria but appeared in danger of losing that vote until the Russian proposal Monday provided a diplomatic opening.
In a speech to the nation Tuesday night, Obama made moral and strategic arguments for taking action on Syria, challenging Congress and the American public to look at video footage of victims of the chemical attack.
Russian President Vladimir Putin responded with an op-ed posted Wednesday night on the New York Times' website, saying "there is every reason to believe" Syrian troops weren't responsible, while challenging Washington and the idea of "American exceptionalism."
His remarks provoked a strong reaction in the United States, with some U.S. politicians deeming them insulting and sickening. But rather than step back, Putin spokesman Dmitry Peskov fired back by claiming Washington was "unaccustomed (to) competition" in global matters and has "grown too used to patting everyone on the back patronizingly."
Other questions loom in Washington.
One has to do with the Syrian opposition, some of whom are already receiving U.S.-funded weapons and ammunition, according to a U.S. official. Yet there's some questions about the mix of moderates and Islamist extremists among the rebels, including some who are affiliated with al Qaeda.
Then there's the issue of Syria's chemical weapons themselves, and whether it's possible to track all of them down.
Some U.S. intelligence analysts believe it's known where most of Syria's stockpile is stored, according to two U.S. officials familiar with internal discussions. But others say the United States might not be able to verify the location of up to 50% of them.