* Russia, Brahimi urge redoubled search for solution
* Moscow meeting brings no peace breakthrough
* Opposition demand for Assad's exit is wrong-Lavrov
By Alissa de Carbonnel
MOSCOW, Dec 29 (Reuters) - Syria faces "hell" if no deal is struck to end 21 months of bloodshed, an international mediator said on Saturday, but his talks in Russia brought no sign of a breakthrough after a week of intense diplomacy.
U.N.-Arab League envoy Lakhdar Brahimi and Russian Foreign Minister Sergei Lavrov both said there was still a chance for a negotiated solution to the conflict, which has killed more than 44,000 people and set world powers against one another.
But Lavrov repeated Russia's stance that President Bashar al-Assad's removal cannot be a precondition for a political solution, saying that such demands were "wrong" and that the opposition's refusal to talk to the government was a "dead end".
Brahimi said: "If the only alternative is really hell or a political process, then all of us must work ceaselessly for a political process. It is difficult, it is very complicated, but there is no other choice."
Lavrov issued a similar exhortation in a joint appearance at an ornate mansion where he meets foreign dignitaries, saying: "The chance for a political settlement remains and it is our obligation to make maximal use of that chance."
But no major new initiatives were announced and Lavrov, whose country has vetoed three United Nations Security Council resolutions meant to put pressure on Assad, gave no indication it would back down from that stance.
"When the opposition says only Assad's exit will allow it to begin a dialogue about the future of its own country, we think this is wrong, we think this is rather counterproductive," he said. "The costs of this precondition are more and more lives of Syrian citizens."
Russia has tried to distance itself from Assad for months and seems to have stepped up its calls for a peaceful resolution as the rebels have gained ground against government forces in the conflict, which began with peaceful protests in March 2011 but which has descended into a civil war.
However, Lavrov noted that Assad has said publicly and privately that he would not go, adding that Russia "does not have the ability to change this".
Brahimi is trying to build on a plan agreed in Geneva in June by the United States, Russia and other powers that called for a transitional government but left Assad's role unclear. The United States said the agreement sent a clear signal that Assad should step down, but Russia said it did nothing of the kind.
"The core of that political process ... is and must be the Geneva agreement," said Brahimi, who took over as the U.N.-Arab League envoy after Kofi Annan quit in frustration at divisions among world powers, chiefly the United States and Russia, and the failure of the Geneva accord to bring a resolution closer.
"There may be one or two little adjustments to make here and there, but it is a reasonable basis for a political process that will help the Syrian people," he said, without elaborating.
TALKING ABOUT TALKS
Brahimi said a plan to resolve the conflict could eventually go to the U.N. Security Council for backing, but only if there was confidence it would be effective.
"What we need to have is a resolution that can work, and I think it is possible to get to that stage if we continue to talk," he said.
The Algerian envoy, who met Assad and others on a five-day trip to Syria this week, is to meet senior U.S. and Russian diplomats together in the coming weeks, after two such meetings this month that produced no signs of a breakthrough.
In Damascus on Thursday, Brahimi called for a transitional government to rule until elections in Syria and said only substantial change would meet demands of ordinary Syrians, but did not specify who could be part of such a body.
A spokesman for the opposition National Coalition said on Friday the coalition "will not negotiate with the Assad regime", and its leader rebuffed Russia's first invitation for talks.
The leader, Moaz Alkhatib, said he would not travel to Moscow and issued conditions for talks, demanding that Lavrov apologise for Russia's support for Assad and that Moscow issue a clear call for him to step down.
Lavrov testily rejected those demands, saying the opposition "should think not of their ambitions but about the Syrian people". Nevertheless, he reiterated Russia's readiness to hold the meeting somewhere outside its territory.
"If they think that Russia can play any kind of role in this drama, then they should meet with us," Lavrov said.
Syria has been a major buyer of Russian arms and hosts a modest naval maintenance facility on the Mediterranean that is Russia's only military base outside the former Soviet Union.
President Vladimir Putin has said Russian vetoes and opposition to U.N. sanctions against Syria are driven by the principle of non-interference in sovereign states. He has accused Washington of using human rights concerns to justify efforts to impose its will around the world.
Putin has emphasised that Moscow will not allow a repeat in Syria of last year's events in Libya, where NATO intervention, authorised by the U.N. Security Council after Russia abstained from a vote, helped rebels topple Muammar Gaddafi.