Russia said Thursday it was ready to discuss a ceasefire in Syria as foreign ministers gathered in Munich in a bid to kick-start peace talks derailed by the regime onslaught on the besieged city of Aleppo.

The UN said 51,000 Syrians had fled the bombardment by government forces, backed by Russian bombers and Iranian fighters, that has left the opposition there virtually surrounded.

Russia's deputy foreign minister Gennady Gatilov said Moscow was ‘ready to discuss the modalities of a ceasefire’ and peace talks between the Syrian government and opposition rebels could ‘possibly start earlier’ than the proposed date of February 25.

The first round of talks in Geneva collapsed earlier this month over the attacks on Aleppo, where observers say at least 500 people have been killed since the offensive began on February 1, and 300,000 people are at risk of being placed under siege.

US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov will host foreign ministers from the 17-nation Syria contact group in Munich, for a meeting billed as a moment of truth for the floundering peace process.

Washington said it wants a ceasefire and humanitarian access to besieged cities but has threatened an unspecified ‘Plan B’ if talks fail, as tension mounts with Moscow over its air campaign.

‘There is no question... that Russia's activities in Aleppo and in the region right now are making it much more difficult to be able to come to the table and to be able to have a serious conversation,’ Kerry said this week.

Russia and Iran say the rebels in Aleppo are just as much ‘terrorists’ as IS and 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.

The rebels say they will not return to talks in Geneva unless government sieges and air strikes end. Kerry met Thursday with the main opposition representative, Riyad Hijab, as well as Saudi Foreign Minister Adel al-Jubeir.

- 'Russia destabilising the West' -

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.

‘For Russia, the war in Syria is about much more than Assad,’ Koert Debeuf, a research fellow at the University of Oxford, told the Carnegie Europe think tank.

‘The goal of Russian President Vladimir Putin is to destabilise and weaken the West. He wants to end the EU's and NATO's attractiveness to countries he considers part of the Russian sphere of influence.’

Nonetheless, experts also say there is a limit to how much Russian aerial bombardment can achieve, particularly as the rebels -- who have the backing of Saudi Arabia and other Gulf countries -- dig in for protracted urban warfare.

‘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.

- Lack of US commitment -

Many have also criticised the United States for not doing more to support the rebels.

Even outgoing French Foreign Minister Laurent Fabius could not hide his frustration as he announced his resignation on Wednesday, saying: ‘You don't get the feeling that there is a very strong commitment’ by the US in Syria.

Washington has been reluctant to involve itself in another war after the quagmires of Afghanistan and Iraq, and ultimately seems more concerned about combatting IS 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... because their objective is to stop the expansion of IS.’

The crisis has also strained relations between Turkey and its Western allies.

Turkish President Recep Tayyip Erdogan hit back at UN calls that Turkey, which is already hosting 2.5 million refugees, should do more for those fleeing Aleppo.

‘We do not have the word 'idiot' written on our foreheads,’ he said. The United Nations should give advice to other countries. And then we can send the refugees to these countries.’

He has also 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’.