By Yara Bayoumy and Steve Gutterman
ALEPPO PROVINCE, Syria/MOSCOW, Dec 28 (Reuters) - Russia invited the leader of Syria's opposition on Friday to visit for the first time, but the opposition swiftly dismissed a renewed call by Moscow for talks with President Bashar al-Assad's government to end the 21-month civil war.
With the rebels advancing over the second half of 2012, diplomats have been searching for months for signs that Assad's main international backer, Moscow, will withdraw its protection.
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," Foreign Minister Sergei Lavrov said on Friday.
That was immediately dismissed by the opposition National Coalition: "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."
But Moscow's Middle East envoy, Deputy Foreign Minister Mikhail Bogdanov, also invited National Coalition leader Moaz Alkhatib to visit, its first such overture to the head of the body formed last month and since recognised by most Western and Arab states as Syria's legitimate representative.
Spokesman Bunni did not say whether Alkhatib would accept the invitation, saying Moscow's intentions were unclear.
U.N. mediator Lakhdar Brahimi, fresh from a five-day trip to Damascus where he met Assad, is due in Moscow for talks on Saturday. Brahimi has been touting a months-old peace plan which calls for a transitional government.
That U.N. plan has long been widely seen as a dead letter, foundering at the outset over the question of whether the transitional body would include Assad or his allies.
Brahimi's predecessor, Kofi Annan, quit in frustration shortly after negotiating it, saying countries were not committed to a deal.
But with rebels having seized control of large sections of the country in recent months, Russia and the United States have been working with Brahimi to resurrect the peace plan as the only internationally recognised diplomatic negotiating track.
Bogdanov said further talks were scheduled between the "three B's" - himself, Brahimi and U.S. Undersecretary of State William Burns.
Speaking in Damascus on Thursday, Brahimi called for a transitional government with "all the powers of the state", a phrase interpreted by the opposition as potentially signalling tolerance of Assad remaining in a ceremonial role.
"This transitional process must not lead to the ... collapse of state institutions. All Syrians, and those who support them, must cooperate to preserve those institutions and strengthen them," Brahimi said.
But such a plan is anathema to the surging rebels, who now believe they can drive Assad out with a military victory, despite long being outgunned by his forces.
"We do not agree at all with Brahimi's initiative. We do not agree with anything Brahimi says," Colonel Abdel-Jabbar Oqaidi, who heads the rebels' military council in Aleppo province, told reporters at his headquarters there.
"We will not allow anyone to trade in the blood of the martyrs of Syria and the sacrifices that Syrians have made by having someone propose any proposal that keeps Bashar al-Assad (in office)."
Oqaidi said the rebels want Assad and his allies tried in Syria for crimes. Assad himself says he will stay on and fight to the death if necessary.
Diplomacy has largely been irrelevant to the conflict so far, with Western states ruling out military intervention like the NATO bombing that helped topple Libya's Muammar Gaddafi last year, and Russia and China blocking U.N. action against Assad.
Meanwhile, the fighting has grown fiercer and more sectarian, with rebels mainly from the Sunni Muslim majority battling Assad's government and allied militia dominated by his Alawite sect, an offshoot of Shi'ite Islam.
Still, Western diplomats have repeatedly touted signs of a change in policy from Russia, which they hope could prove decisive, much as Moscow's withdrawal of support for Serbian leader Slobodan Milosevic heralded his downfall a decade ago.
Bogdanov said earlier this month that Assad's forces were losing ground and rebels might win the war, but Russia has since rowed back, with Lavrov last week reiterating Moscow's position that neither side could win through force.
Still, some Moscow-based analysts see the Kremlin coming to accept the need to adapt its position to the possibility of rebel victory.
"As the situation changes on the battlefield, more incentives emerge for seeking a way to stop the military action and move to a phase of political regulation," said Dmitry Trenin, director of the Carnegie Moscow Center.
"I think that not only Moscow - which has so far been holding onto the idea that there were enough resources for Assad to hang on for a long time - is beginning to understand this, but also Damascus."
Meanwhile, on the ground the bloodshed that has killed some 44,000 people continues unabated. According to the pro-opposition Syrian Observatory for Human Rights, a monitoring group based in Britain, 150 people were killed on Thursday, a typical toll as fighting has escalated in recent months.
Government war planes bombarded the town of Assal al-Ward in the Qalamoun district of Damascus province for the first time, killing one person and wounding dozens, the observatory said.
The bombardment may have included areas in Qalamoun from which the army withdrew on Thursday, the observatory said. Its accounts could not be verified.
In Aleppo, Syria's northern commercial hub, clashes took place between rebel fighters and army forces around an air force intelligence building in the Zahira quarter, a neighbourhood that has been surrounded by rebels for weeks.
Most of the dead have been civilians. Both sides have committed atrocities, although the United Nations says government forces and their allies have been more culpable.
Footage uploaded to the internet on Friday showed young men beating the bloody corpse of another man with a stick. One reaches down with a knife and gleefully slices off an ear.
Opposition activists said the footage showed government-allied militia members desecrating bodies, but its provenance could not be confirmed.