Britain and the US may soon be pressured to consider clemency for President Bashar al-Assad as part of a new political transition package for Syria.
The idea may be discussed at a peace conference in Geneva tentatively planned for the end of this month, sources said last night.
A clemency offer to Mr Assad, protecting him from prosecution by the International Criminal Court, could involve him being allowed to flee Syria for another country, possibly Russia or Iran. Asked specifically if that might be part of an eventual deal, the UK source said that it was "a question you have to look at". Officials insisted, however, that David Cameron has not yet taken a view on the matter.
After meetings about Syria involving leaders of most of the main players including the Russian President, Vladimir Putin, at this week's G20 summit in Mexico, there is cautious optimism that the proposed Geneva meeting might now go ahead.
It would be attended by opposition leaders from Syria, the permanent five of the UN Security Council, Turkey and Saudi Arabia. The Syrian regime would also have to be at the table. The tentative plan could still be tripped up, however, notably by a Russian proposal that Iran should also be invited. "That is a red line for us," a Downing Street source indicated.
The alternative, the source said, would be to return to the UN Security Council to try to pass a new resolution that would allow for much tougher international sanctions. Any such effort in New York would be likely to founder, however, on opposition from Russia and China.
The optimal scenario is that a Geneva meeting would set in train a process likely to take 18 to 24 months that would see a new government put in place, without Assad at the top but possibly with some current members still remaining and representatives from the opposition sides. Eventually elections in Syria would be held.
Both the British and the Americans have come away from Los Cabos marginally encouraged that Russia, for whom Assad has been an important ally, is more open than before to the notion of him eventually leaving.
While some Russian officials attempted to deny this yesterday, a source from Downing Street insisted that this was the impression given to Mr Cameron during a 50-minute meeting with Mr Putin on Tuesday.
Meanwhile in Syria, at least 20 soldiers were reported killed during fierce gun battles in the east. The fighting came as the International Committee of the Red Cross said it was preparing to evacuate people trapped in the old city of Homs, the rebel stronghold which has been intensely shelled for much of the past six months. Dozens of families have been caught in crossfire between government forces and rebels, though yesterday both sides had reportedly agreed to a temporary truce to allow aid workers access.