Russia and Syria embraced Secretary of State John F. Kerry’s suggestion Monday that the Syrian government could avert a U.S. attack by placing its chemical weapons under international control, upending the Obama administration’s efforts to sharpen its case for military action.

U.S. officials said Kerry’s comment, made in response to a question at a news conference in London, was not intended to be a diplomatic opening. But Kerry’s Russian and Syrian counterparts quickly followed up, and the idea drew immediate interest internationally and from top Democrats in Washington.

By the end of the day, President Obama conceded that the idea of monitoring and ultimately destroying Syria’s arsenal “could potentially be a significant breakthrough.” The Senate postponed a vote scheduled for Wednesday on whether to back a proposed punitive strike.

“I think you have to take it with a grain of salt, initially,” Obama said in an interview with NBC that was among several he gave Monday in pursuit of public backing for a military strike in response to an alleged Aug. 21 gas attack on Syrian civilians.

“We are going to run this to ground,” Obama said. “We’re going to make sure that we see how serious these proposals are.”

See all six of Obama’s interviews about Syria.

The president plans to address the nation Tuesday evening in a speech originally planned to be the capstone of a newly focused moral and political case to rally a skeptical public and reluctant lawmakers.

The timing of the new proposal was awkward and its apparent genesis perhaps more so.

It began when Kerry was asked early Monday whether Assad could avoid a U.S. attack.

“Sure. He could turn over every bit of his weapons to the international community within the next week, without delay,” Kerry responded with a shrug. “But he isn’t about to.”

As Kerry flew back to Washington to help lobby lawmakers, he received a midair call from Russian Foreign Minister Sergei Lavrov, who said he had heard the secretary’s remarks and was about to make a public announcement.

The statement in Moscow came before Kerry landed.

“We are calling on the Syrian authorities [to] not only agree on putting chemical weapons storages under international control but also for its further destruction and then joining the Organization for the Prohibition of Chemical Weapons,” Lavrov said, adding, “We have passed our offer to [Syrian Foreign Minister] Walid al-Moualem and hope to receive a fast and positive answer.”

Moualem, who was in Moscow meeting with Lavrov, followed with a statement that his government “welcomes Russia’s initiative, based on the Syrian government’s care about the lives of our people and security of our country.”

Although Syrian President Bashar al-Assad denies having a stockpile of the widely banned weapons, the idea of international control also quickly gained traction among diplomats and at least some senior Democrats whose support Obama seeks for a show of force.

Sen. Dianne Feinstein (D-Calif.) was the first senior lawmaker to voice support for the Russian proposal.

“I think if the U.N. would accept the responsibility of maintaining these facilities, seeing that they’re secure, and that Syria would announce that it is giving up any chemical weapons programs or delivery system vehicles that may have been armed, then I think we’ve got something,” Feinstein said.

The Russian announcement was met with approval by international backers and critics of a U.S. strike. U.N. Secretary General Ban Ki-moon, who has said a U.S. attack on Syria would be illegal without U.N. approval, signaled support, as did British Prime Minister David Cameron.

French Foreign Minister Laurant Fabius, whose government had said it would join an American attack and who two days ago stood at Kerry’s side in Paris to pledge an all-out effort to build public support, said it was worth testing. German Chancellor Angela Merkel, who has been wary of a strike, welcomed the idea.

Republican Sens. John McCain (Ariz.) and Lindsey O. Graham (S.C.) said the proposal came only because Assad feels the threat of military force and that Congress should continue considering Obama’s request for legislative backing. But the two said the proposal should be given a chance — and a test of its sincerity — by being committed to writing in a U.N. Security Council resolution.

“We should not trust, and we must verify,” the pair said in a joint statement.

A senior State Department official said Kerry warned Lavrov that the United States was “not going to play games.”

Current and former Obama administration officials scrambled Monday to say the proposal should not derail plans for a punitive strike. They suggested it was a delaying tactic after more than two years of diplomatic efforts with Syria and its ally Russia, albeit one spurred by the prospect that a U.S. military attack is imminent.

“It’s very important to note that it’s clear that this proposal comes in the context of the threat of U.S. action and the pressure that the president is exerting,” deputy national security adviser Tony Blinken said at the White House. “So it’s even more important that we don’t take the pressure off and that Congress give the president the authority he’s requested.”

Obama said in an interview on “PBS NewsHour” Monday that he had discussed the possibility of international monitoring with Russian President Vladi­mir Putin at last week’s Group of 20 summit in St. Petersburg.

The senior State Department official said Lavrov had previously discussed the idea in conversations with Kerry, including a telephone call as recently as Thursday, but never in the context of the proposed U.S. military action.

The official, who requested anonymity to detail the internal discussions, said Lavrov told Kerry about the new proposal in a telephone call while Kerry flew home Monday.

The Russian announcement came as several top Obama administration officials were fanning out across Washington in a coordinated lobbying effort.

Inside an hour on Monday afternoon, Blinken, deputy national security adviser Benjamin Rhodes, national security adviser Susan E. Rice, White House press secretary Jay Carney and State Department spokeswoman Marie Harf all made the same argument.

“Failing to respond to this brazen attack could indicate that the United States is not prepared to use the full range of tools necessary to keep our nation secure,” Rice said in an address at the New America Foundation. “Any president, Republican or Democrat, must have recourse to all elements of American power to design and implement our national security policy, whether diplomatic, economic or military.”

In Washington, former secretary of state Hillary Rodham Clinton said after a meeting with Obama that if Syria immediately surrenders its chemical weapons, “that would be an important step, but this cannot be another excuse for delay or obstruction.”

Last month’s reported chemical attack, which the United States says killed more than 1,400 civilians, brought worldwide condemnation, as well as vows of military action by Obama, who had previously described the use of such banned weapons as a “red line.”

Gen. Salim Idriss, chief of staff of the rebel Free Syrian Army, described the Syrian government’s acceptance of the Russian proposal as a “new lie” aimed at heading off intervention.

DeYoung reported from London. Englund reported from Moscow