Barack Obama welcomed a Russian proposal on Monday for Syria to place its chemical weapons under international control, opening up the first real chance of a political settlement to the crisis since hundreds of civilians died in an attack on a Damascus suburb last month.

In a series of primetime television interviews, Obama described Russia's offer as a "possible breakthrough" and a "potentially positive development" in the standoff with the regime of Bashar al-Assad. With the prospect of a deal with the Syrians in the offing, the Senate majority leader Harry Reid postponed a crucial vote to authorise military action. Obama conceded in an NBC interview on Monday night that he might lose his campaign in Congress for authorisation. "I wouldn't say I'm confident" of the outcome, he said, adding that he had not decided what to do if it voted against him.

Russia's proposal came after an apparent stumble by the US secretary of state, John Kerry, which set off a diplomatic scramble in Washington as administration officials sought to assess whether it offered a way out for Obama from what has become an increasingly intractable problem.

Speaking in London, Kerry suggested that the only way for Syria to avoid the threat of a US attack would be for it to hand over all its chemical weapons within a week. The remarks were characterised as a blunder by some Washington commentators, and the Department of State at first attempted to play down their significance, saying Kerry had been speaking "rhetorically" about a situation that was unlikely to materialise.

But the comments were immediately seized on by the Russian foreign minister Sergey Lavrov, who raised the prospect of international observers supervising such a handover. "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," Lavrov said after a meeting with his Syrian counterpart, Walid al-Moallem.

Intentional or not, Kerry's comments opened up a chance to defuse the crisis at a moment when Obama was already struggling to persuade Congress of the need for US intervention. In his NBC interview, the president said: "You have to take this with a grain of salt initially, but between the statements that we saw from the Russians, the statement today from the Syrians, this represents a potentially positive development."

Obama said that the administration would work to assess the seriousness of the proposals. "We are going to run this to ground. John Kerry will be talking to his Russian counterpart. We're going to make sure that we see how serious these proposals are," Obama said.

But US officials nevertheless expressed scepticism over whether Syria would follow through. "Unfortunately, the track record to date does not inspire a lot of confidence," said US deputy national security adviser Tony Blinken.

Obama also claimed that he had first discussed the idea at the G20 summit as his administration scrambled to claim credit for the Russian deal and insisted Syria was responding to US pressure. "It is unlikely that we would have arrived at that point without a credible military threat," Obama told CNN.

The president will address the American people in a direct televised broadcast on Tuesday evening. By that time, the White House will have had the opportunity to assess the viability of the Russian proposal. But already on Monday night there was a clear sense of relief on Capitol Hill, where support for military action had been patchy.

A key legislative ally of Obama, Senator Dianne Feinstein of California, the chairwoman of the Senate intelligence committee, said she would welcome a move by Syria to put chemical weapons beyond use. "I believe that Russia can be most effective in encouraging the Syrian president to stop any use of chemical weapons and place all his chemical munitions, as well as storage facilities, under United Nations control until they can be destroyed," Feinstein said.

The former US secretary of state, Hillary Clinton, speaking after a hastily arranged meeting with Obama at the White House, where she was due to speak at an event about illegal wildlife trafficking, said the move could represent an "important step". In her first comments about the Syria crisis, Clinton warned that it could not make "another excuse for delay or obstruction".

Kerry later spoke to Lavrov by phone and Washington scrambled to place its own spin on the unexpected developments. Jay Carney, the White House spokesman, insisted that the offer by Russia and Syria had only come about because of "sustained pressure" from the US.

"It is our position, and has been for some time, that the Syrian regime should not use and also not possess stockpiles of chemical weapons, and we would welcome any proposals that would result in the international control and destruction of that chemical weapon stockpile," he said at a White House briefing.

"There is no question that we have seen some indications of an acceptance of this proposal [from the Syrians], but this is a very early stage and we approach this with scepticism," he added.

The proposal was welcomed by the UN and a number of European governments. UN secretary general Ban Ki-moon said he would propose the security council unite and vote on an immediate chemical weapons transfer, placing weapons and chemical precursors in a safe place within Syria for international destruction.

Earlier, Ban said that he hoped that a forthcoming report by UN inspectors on the 21 August attack on a rebel-held areas east of Damascus, which the US says killed more than 1,400 people, would spur the international community into action.

"Two and half years of conflict in Syria have produced only embarrassing paralysis in the security council," Ban said at a press conference.

The French government has said it would wait for the UN report, being prepared by a Swedish scientist, Åke Sellström, before making a final decision on taking part in military action.

The Sellström report is unlikely to come before the end of this week, diplomatic sources said. The samples brought back from a two-week visit are being studied in four European laboratories, to ensure that the result is conclusive.

On Tuesday, Human Rights Watch said it had concluded that Syrian government forces were behind the poison gas attack. The US-based group said based this on witness accounts, information on the likely source of the attacks, remnants of the weapons used and medical records of victims.

In the British parliament, David Cameron responded positively but cautiously to Russia's move, saying if it was a genuine offer, it should be regarded as a big step forward.

Downing Street initially indicated that the Kerry proposal was not serious, pointing out that the idea had not been raised during the lengthy discussion on Syria at the G20 dinner in St Petersburg. They added that the focus should be on Assad's record with chemical weapons.

But in a Commons debate on the G20 and Syria, Cameron said it would be "hugely welcome" if the Assad regime were to hand over its chemical weapons stockpile.

Susan Rice, the US national security adviser, said that "even greater barbarism" would follow if the US did not take military action against Assad. "The decision our nation makes in the coming days is being watching in capitols around the world, especially in Teheran or Pyongyang," Rice told an audience at the New America Foundation in Washington on Monday.

Rice, the former US ambassador to the UN, did not address Russia's offer for Assad to relinquish his chemical stockpiles.

Additional reporting by Spencer Ackerman in Washington and Patrick Wintour in London