The suggestion to secure the chemical weapons "could potentially be a significant breakthrough," Obama told NBC News in another interview. "But we have to be skeptical because this is not how we've seen them operate over the last couple a years."
He cast Russia's proposal as a direct result of the pressure being felt by Syria because of the threat of a U.S. strike and warned that he would not allow the idea to be used as a stalling tactic.
"I don't think that we would have gotten to this point unless we had maintained a credible possibility of a military strike, and I don't think now is the time for us to let up on that," he said.
Still, the White House has had scant success in persuading members of Congress — including Democrats — to support the idea of military action. Senators continued to announce their opposition through the day.
The proposal from Russian Foreign Minister Sergey Lavrov came just hours after Kerry told reporters in London that Assad could avoid a U.S. attack and resolve the crisis surrounding the use of chemical weapons by surrendering control of "every single bit" of his arsenal to the international community by the end of the week.
The State Department sought to tamp down the potential impact of Kerry's comments by calling them a "rhetorical" response to a hypothetical question and not "a proposal." But their importance became more clear as the day progressed.
Kerry spoke by phone with Lavrov shortly after making his comments in London, and officials familiar with the call said Lavrov had told Kerry that he had seen the remarks and would be issuing a public statement. Kerry told Lavrov that his comments were not a proposal but the U.S. would be willing to review a serious plan, the officials said. They stressed that he made clear that Lavrov could not present the idea as a joint U.S.-Russian proposal.