Foreign ministers were locked in late-night talks in Munich on Thursday, seeking a way to revive a floundering Syrian peace process as Russia warned of the possibility of a "new world war".
An onslaught on the key rebel stronghold of Aleppo by Syrian troops, backed by Russian bombers and Iranian fighters, derailed peace talks earlier this month and has triggered an exodus of over 50,000 refugees.
Russian Foreign Minister Sergei Lavrov said he had "made propositions for a ceasefire that are quite specific" as he sat down for talks with US counterpart John Kerry.
But the mood in Munich was downbeat, with rumours that Russia had only offered a ceasefire from March 1, giving another three weeks for an offensive which the UN says could place 300,000 people under siege.
The bombardments on Aleppo have left the opposition there virtually encircled and observers say 500 people have died since they began on February 1 -- the latest hellish twist in a five-year war that has claimed some 250,000 lives.
Russian Prime Minister Dmitry Medvedev, meanwhile, warned that any move by Gulf nations to send in troops to support the rebels would risk a "new world war".
"The Americans and our Arabic partners must think hard about this: do they want a permanent war?" he told Germany's Handelsblatt newspaper.
- 'Russia intensifying conflict' -
US diplomats hit back at Russia's support for Syrian President Bashar al-Assad and said any ceasefire should be "immediate".
"It has been Russian support for the Assad regime over the past months, and most recently in the siege on Aleppo, that has exacerbated, intensified the conflict," said State Department spokesman Mark Toner.
French President Francois Hollande added his voice to the criticism.
"Helped by the Russians, (Assad) is massacring some of his own people, even if he is also taking action against a certain number of terrorists. I ask that Russia's actions stop," Hollande said.
Russia and Iran have repeatedly labelled the rebels in Aleppo as "terrorists" and suggested there can be no settlement until they have been militarily defeated.
"Those who are outside Syria should help the peace process and not seek to impose conditions on the Syrian people," Iranian Foreign Minister Javad Zarif told Iran's state TV after arriving for the talks.
- US 'Plan B' -
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A first round of talks between the Syrian government and the opposition in Geneva collapsed earlier this month over the attacks on Aleppo.
The rebels say they will not return to talks, pencilled in for February 25, unless government sieges and air strikes end.
Washington has threatened an unspecified "Plan B" if talks fail, as tension mounts with Moscow over its air campaign.
The two sides traded accusations on Thursday about bombing in Aleppo, with the Pentagon claiming two hospitals had been destroyed, and Moscow saying US planes had struck the city -- which was flatly denied by Washington.
Analysts see little hope of reconciling differences.
Syria is a crucial ally and military staging post for Russia and Iran, while a growing number of observers say Moscow has benefited from the chaos created by the war, particularly the refugee crisis in Europe.
"The goal of Russian President Vladimir Putin is to destabilise and weaken the West," Koert Debeuf, a research fellow at the University of Oxford, told the Carnegie Europe think tank.
But they also see little chance of a decisive victory for Syrian President Bashar al-Assad.
"The idea of a full reconquest... seems neither credible nor durable. It will simply turn into a terrorist or guerrilla situation," said Camille Grand, of the Foundation for Strategic Research in Paris.
- Strained relations -
Many have criticised the United States for not doing more to support the rebels.
Washington has been reluctant to involve itself in another war after the quagmires of Afghanistan and Iraq, and has sought to focus more on combatting the Islamic State group than getting involved in the civil war between Syria's regime and rebels.
"The US has given up the idea of toppling Assad," said Grand. "Kerry seems willing to accept pretty much anything to resolve the crisis."
The conflict has also strained relations between Turkey and its Western allies.
Turkish President Recep Tayyip Erdogan has slammed Washington's increasingly close alliance with the Kurdish militias in the fight against IS, saying it was turning the region into "a pool of blood".