At the State Department, spokeswoman Marie Harf said the U.S. would consider the Russian proposal only with "serious skepticism" because it might be merely a stalling tactic. She said Syria had consistently refused to destroy its chemical weapons in the past.
In fact, she said the developments made it even more important for Congress to authorize the use of force against Syria as a means for pushing Assad to actually get rid of chemical weapons stocks.
The U.S. accuses Assad's government of being behind an attack using sarin gas in a Damascus suburb on Aug. 21, killing 1,429 people. Some other estimates of the deaths are lower, but there is wide agreement that chemical weapons were used. Experts believe that the Syrian government's arsenal of chemical weapons includes nerve agents like sarin, tabun and VX as well as mustard gas.
In an interview with Charlie Rose that was broadcast Monday on "CBS This Morning," Assad denied responsibility for the Aug. 21 attack, accused the Obama administration of spreading lies without providing a "single shred of evidence," and warned that air strikes against his nation could bring retaliation. Pressed on what that might include, Assad responded, "I'm not fortune teller."
Later Monday, Syria's foreign minister, meeting with his Russian counterpart in Moscow, addressed the idea of getting rid of any chemical weapons.
"Syria welcomes the Russian proposal out of concern for the lives of the Syrian people, the security of our country and because it believes in the wisdom of the Russian leadership that seeks to avert American aggression against our people," said al-Moallem.
U.S. officials said they were taken aback by Kerry's comments, which came at a news conference with British Foreign Secretary William Hague and in response to a question about what, if anything, Assad could do to stop the U.S. from punishing it for the use of chemical weapons.
"Sure," Kerry replied. "He could turn over every single bit of his chemical weapons to the international community in the next week. Turn it over, all of it, without delay, and allow a full and total accounting for that. But he isn't about to do it, and it can't be done, obviously."
Traveling with Kerry, State Department spokeswoman Jen Psaki tried to blunt the suggestion.
"Secretary Kerry was making a rhetorical argument about the impossibility and unlikelihood of Assad turning over chemical weapons he has denied he used," she said. "His point was that this brutal dictator with a history of playing fast and loose with the facts cannot be trusted to turn over chemical weapons otherwise he would have done so long ago."
And, in a speech, Obama's national security adviser Susan Rice reiterated that the president had decided it is in U.S. interests to carry out limited strikes.
Al-Moallem and Lavrov had not reacted to Kerry's comments when they spoke to reporters immediately after their meeting. But Lavrov appeared before television cameras several hours later to say Moscow would urge Syria to quickly place its chemical weapons under international control and then dismantle them.
"If the establishment of international control over chemical weapons in that country would allow avoiding strikes, we will immediately start working with Damascus," Lavrov said.
"We are calling on the Syrian leadership to not only agree on placing chemical weapons storage sites under international control, but also on its subsequent destruction and fully joining the treaty on prohibition of chemical weapons," he said.
Russia's proposal provided confirmation from Syria's most important international ally that the Syrian government possesses chemical weapons, and al-Moallem's welcome was a tacit acknowledgment. Syria's foreign ministry last year retracted a threat to use chemical weapons, saying it was not acknowledging that it had them.