British Prime Minister David Cameron meets Russian President Vladimir Putin on Sunday for key talks about the conflict in Syria which could set the tone for the G8 summit next week.
Cameron will seek to forge a consensus on how to deal with Syria when he hosts the leaders of the world's most industrialised nations in Lough Erne, Northern Ireland, from Monday.
Washington has upped the ante on Syria by vowing to send military aid to rebel forces trying to unseat President Bashar al-Assad after saying it had proof that the regime had crossed a "red line" by using chemical weapons on a small scale.
Russia, which has given the Syrian regime military support and ignored months of pleas from the West to rein in Assad, was dismissive of the US claims.
American officials will not reveal exactly what military support will go to the rebels' Supreme Military Council, although by many estimates it will initially be assault rifles and ammunition.
It seems unlikely it will include the imposition of a no-fly zone to protect rebels from air attack. Russian Foreign Minister Sergei Lavrov said on Saturday that such a zone would violate international law.
Lavrov backed Assad, saying there was no need for the Syrian leader to use chemical weapons against the rebels because his forces were making steady advances on the ground anyway.
"What sense is there for the regime to use chemical arms -- especially in such small amounts?" he asked.
Ahead of his talks at Downing Street with Putin, Cameron said it was essential to assist the moderate rebels prepared to work with the West before extremists linked to al-Qaeda gained the upper hand in the opposition.
He told Sky News: "I want to help the Syrian opposition to succeed and my argument is this: Yes, there are elements of the Syrian opposition that are deeply unsavoury, that are very dangerous, very extremist, and I want nothing to do with them.
"But there are elements of the Syrian opposition who want to see a free, democratic, pluralistic Syria that respects the rights of minorities, including Christians, and we should be working with them."
Cameron has not said whether he favours sending weapons to the rebels, but he believes the lifting of a European Union arms embargo was essential to put pressure on Assad.
As tensions over rise over a conflict that has cost more than 90,000 lives, Putin will follow his talks with Cameron at Downing Street on Sunday by meeting President Barack Obama at the G8 summit on Monday.
Cameron held a teleconference on Syria with four of the G8 leaders on Friday -- Obama, French President Francois Hollande, German Chancellor Angela Merkel and Italian Prime Minister Enrico Letta.
A statement from Cameron's office said the leaders had discussed "how G8 countries should all agree to work on together a political transition to end the conflict."
The pre-G8 meetings are also likely to ponder the impact of the election of moderate cleric Hassan Rowhani as Iran's new president.
While Syria is set to dominate the G8 agenda, Cameron made progress on Saturday with another of his summit aims by striking a deal with British overseas territories to clamp down on tax evasion.
The agreement with Bermuda, the British Virgin Islands, the Cayman Islands, Gibraltar, Anguilla, Montserrat, the Turks and Caicos Islands, Jersey, Guernsey and the Isle of Man strengthens Cameron's hand going into the talks in Lough Erne.
Many observers regard the islands as tax havens, although they reject such descriptions.
AFP