U.N.- Arab League envoy Lakhdar Brahimi said in Moscow that responsible people inside and outside Syria should "help the Syrians stop their descent into more and more bloodshed, into more and more chaos and perhaps a failed state".
Efforts to find a negotiated solution to a 21-month-old war that has killed some 44,000 people have floundered, with the opposition, buoyed by rebel military advances, demanding that Assad be excluded from power before any talks can proceed.
In a sign that the war may not quickly be won, government forces - in retreat for much of the past few months - scored a victory in the strategically important central city of Homs, where they pushed rebels from a district after days of fighting.
But in the north, Syria's national airline had to cancel a flight from Cairo to Aleppo, according to Egyptian airline officials, due to insecurity at an airport which rebels have declared as a target and where explosions were heard overnight.
"If the only alternative is really hell or a political process, then all of us must work ceaselessly for a political process," Brahimi said in Moscow. "It is difficult, it is very complicated but there is no other choice."
Western and some Arab states which back the revolt against Assad are hoping that Russia, his main international protector and arms supplier, will drop its support.
They have been searching for signs that Moscow, an ally of Syria since Assad's father seized power 42 years ago, is changing its stance - so far mostly in vain.
After meeting Brahimi, Russian Foreign Minister Sergei Lavrov firmly repeated Moscow's position that Assad's removal cannot be a precondition for negotiations, calling the Syrian opposition's refusal to talk to Damascus a "dead end".
"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."
The United States and its allies hope a change of heart in Moscow could prod Assad to yield power, much as Russia's withdrawal of support for Serbian leader Slobodan Milosevic heralded his downfall a decade ago.
Lavrov noted that Assad has repeatedly said he would not go, adding that Russia "does not have the ability to change this".
Brahimi's peace plan has stalled on the demand by the opposition that Assad be excluded from any transitional government, a precondition also backed by the United States, European countries and most Arab states.
Egypt's President Mohamed Mursi, repeating the most populous Arab country's public support for the rebellion, said there was "no place for the current regime in the future of Syria".
"The revolution of the Syrian people, which we support, will go forward, God willing, to realise its goals of freedom."
Most Arab states are ruled by Sunni Muslims, who form the majority in Syria and are the foundation of the revolt against Assad, a member of the Shi'ite-linked Alawite minority sect.
Brahimi's plan was formally agreed in Geneva in June by world powers, but Washington and Moscow argued from the outset over the core question of whether the plan meant Assad must go.
In Damascus, Brahimi advocated a transitional government "with all the powers of the state", but his wording did not exclude a role for Assad, a red line for the opposition.
The envoy's credibility with the rebels appears to have withered. In the rebel-held town of Kafranbel, demonstrators held up banners ridiculing Brahimi with English obscenities.
"We do not agree at all with Brahimi's initiative. We do not agree with anything Brahimi says," the rebel chief in Aleppo province, Colonel Abdel-Jabbar Oqaidi, said on Friday.
OPPOSITION SNUB FOR MOSCOW
Moscow has invited the main opposition leader, Moaz Alkhatib, to visit for talks, but Alkhatib rejected the invitation outright on Friday, instead demanding Lavrov apologise for Russia's support of Assad. He did, however, say he could meet Russian officials in a third country.
Brahimi said a political solution had to be based on the Geneva agreement negotiated by his predecessor, Kofi Annan, shortly before Annan quit in frustration at the divisions among veto-wielding powers on the U.N. Security Council.
"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," Brahimi said of the Geneva plan.
Brahimi is to meet senior U.S. and Russian diplomats together in the coming weeks. Two such meetings this month produced no signs of a breakthrough.
Having seized much of northern and eastern Syria over the last six months, Assad's opponents seem even less likely to accept talks with the government now than when the Geneva agreement first flopped in June.
Rebels say they expect to win the war on the ground. But if both sides intend to fight to the bitter end, the longest and deadliest war to have emerged from last year's Arab revolts may have a long time left to run its course.
Despite its setbacks, the government still has the bigger arsenal and a potent air force. It controls most of the densely-populated southwest of Syria, the Mediterranean coast, most of the main north-south highway and military bases countrywide.
Life is hard in rebel-held areas, without state funding or electricity and with a harsh winter setting in.
Government forces pushed insurgents from the Deir Ba'alba district of the central city of Homs after several days of fierce fighting, opposition activists acknowledged on Saturday.
Homs, which controls the strategically vital highway linking Damascus to the Alawite heartland on the coast, was the scene of some of the worst violence of the war in early 2012.
There were unconfirmed reports that dozens of rebel fighters had been killed in the battle for Deir Ba'alba, said Rami Abdelrahman, director of the British-based, pro-opposition Syrian Observatory for Human Rights, a monitoring group.