Britain and America are willing to offer the Syrian president, Bashar al-Assad, safe passage – and even clemency – as part of a diplomatic push to convene a UN-sponsored conference in Geneva on political transition in Syria.
The initiative comes after David Cameron and Barack Obama received encouragement from Russia's President Vladimir Putin in separate bilateral talks at the G20 in Mexico.
A senior British official said: "Those of us who had bilaterals thought there was just enough out of those meetings to make it worth pursuing the objective of negotiating a transitional process in Syria."
With daily reports of civilian deaths and the conflict apparently taking on an increasingly sectarian hue, Britain is willing to discuss giving clemency to Assad if it would allow a transitional conference to be launched. He could even be offered safe passage to attend the conference.
One senior UK official said: "It is hard to see a negotiated solution in which one of the participants would be willing voluntarily to go off to the international criminal court." It was stressed Cameron had not made a final decision on the matter.
During talks at the G20, British and American officials were convinced Putin was not wedded to Assad remaining in power indefinitely, although even this limited concession is disputed in Moscow.
On the basis of these discussions, the US secretary of state, Hillary Clinton, will now seek to persuade the former UN secretary general, Kofi Annan, to change the format of his plans to construct a contact group on Syria, and instead host a conference using the transition on Yemen as the model.
In the case of Yemen, the president, Ali Saleh, was granted immunity in February despite the massacre of civilians. His deputy, to whom he ceded power, is drawing up a new constitution.
Participants would include representatives of the Syrian government, leading figures in the opposition, the five permanent members of the UN security council and key figures in the region, such as Turkey and Saudi Arabia. Russia has been pressing for Iran to be able to attend.
The meeting, under Annan's chairmanship, would be held by the end of the month with an objective of establishing a broader-based government leading to elections in 18 months time.
British officials said: "We do not think it makes sense to invite the Iranians for a number of reasons. We are under no illusions about this and are entirely realistic about the prospects of this happening. It may come off. It could capsize on whether Iran gets invited or not. But it is worth a try given the gravity of events there."
Cameron said on Tuesday Syria was "in danger of descending into a bloody civil war" and there was little time left to act.
As an alternative the US might go for a tougher UN security council resolution on sanctions, but the prospect of a no-fly zone, overcoming Russian objections, is not regarded as realistic.
It also emerged that Cameron confronted Putin over arms supplies and had been personally involved in plans to prevent a Russian-manned shipment of three repaired attack helicopters and air defence systems reaching Syria.
The ship, the MV Alead, returned to Russia after UK insurance was withdrawn on Monday.
It emerged that Cobra, the government emergency committee, held secret sessions last Thursday, Friday and Monday at which options to stop the shipment were discussed, including discussions with the Dutch government to stop the ship on the basis that it was flying under the Dutch Antilles flag.
Cameron was updated on the process whileat the G20 summit in Mexico and had at one point been willing to consider ordering the ship to be boarded had it continued down the English Channel.
Russia is not party to any arms embargo and claims the opposition are being armed by the Saudis.
US and UK intelligence had identified the cargo on the ship as well as false documention about its destination. The ship turned back after insurance was withdrawn following UK government pressure. The EU arms embargo on Syria will now be tightened up to cover insurance.
The article on the web site of the Guardian