Russia opened up a possible diplomatic solution to the Syrian chemical weapons crisis on Monday with a pledge to persuade the Assad regime to hand over its chemical arsenal to international supervision to be destroyed.

Russia's new initiative was announced by its foreign minister, Sergey Lavrov, hours after the US secretary of state, John Kerry, suggested that the Syrian government could avert punitive US air strikes in retaliation for an alleged chemical attack on 21 August, if it surrendered "every single bit" of its arsenal by the end of the week.

However, Kerry added that Assad "isn't about to do it", and the state department hastily issued a clarification saying that apparent ultimatum was "rhetorical" rather than a concrete bargaining position.

But Lavrov appeared to seize on the idea as a means of averting US military intervention.

"If the establishment of international control over chemical weapons in that country would allow avoiding strikes, we will immediately start working with Damascus," he 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.

He added that he has already handed over the proposal to Moallem and expected a "quick, and, hopefully, positive answer". Moallem was quoted by the French Press Agency as welcoming the Russian proposal.

Both ministers said they looked forward to publication of a report by UN weapons inspectors on the 21 August attack on a rebel-held area east of Damascus called Ghouta, which the US says killed more than 1,400 people.

The French government has also 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.

Sellström only has a mandate to state whether chemical weapons were used, not who used them. However, his report will include interviews with survivors and observations on the missiles or other delivery systems used in what the UN is saying will be an "evidence-based narrative" of the attack.

"Should Dr Sellström's report confirm the use of chemical weapons, then this would surely be something around which the security council could unite in response – and indeed something that should merit universal condemnation, Ban Ki-moon, the UN secretary general, said on Monday.

"I am already considering certain proposals that I could make to the security council when presenting the investigation team's report. There would be a need for accountability, both to bring to justice those who used them – should Dr Sellström confirm their use – and to deter anyone else from using these abhorrent methods of warfare. There would be a need for greater security regarding any chemical weapons stocks."

In the UK 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.

Number 10 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 Saint Petersburg. They added 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.