Syria's opposition leader has rejected an invitation from Russia for peace talks, dealing another blow to international hopes that diplomacy can be resurrected to end a 21-month civil war, Reuters said.

Russia, President Bashar al-Assad's main international protector, said on Friday, December 28 it had sent an invitation for a visit to Moaz Alkhatib, whose six-week-old National Coalition opposition group has been recognized by most Western and Arab states as the legitimate voice of the Syrian people.

But in an interview on Al Jazeera television, Alkhatib said he had already ruled out such a trip and wanted an apology from Moscow for its support for Assad.

With the rebels advancing steadily over the second half of 2012, diplomats have been searching for months for signs that Moscow's willingness to protect Assad is faltering.

So far Russia has stuck to its position that rebels must negotiate with Assad's government, which has ruled since his father seized power in a coup 42 years ago.

"I think a realistic and detailed assessment of the situation inside Syria will prompt reasonable opposition members to seek ways to start a political dialogue," Lavrov said on Friday.

That was immediately dismissed by the opposition: "The coalition is ready for political talks with anyone ... but it will not negotiate with the Assad regime," spokesman Walid al-Bunni told Reuters. "Everything can happen after the Assad regime and all its foundations have gone. After that we can sit down with all Syrians to set out the future."