The European Union accused Russia on Monday of putting peace efforts at risk in Syria, calling on Moscow to stop bombing Western-backed rebels but failing to agree on a role for President Bashar al-Assad in resolving the crisis.

Seeking a common front in their criticism of Russia's dramatic military intervention, EU foreign ministers warned that air strikes designed to support Assad could also deepen the 4-1/2-year civil war that has killed 250,000 people.

"The recent Russian military attacks ... are of deep concern and must cease immediately," ministers said in their most strongly-worded statement on Russia's intervention.

"The military escalation risks prolonging the conflict, undermining a political process, aggravating the humanitarian situation and increasing radicalisation," said the ministers, who are meeting in Luxembourg.

EU leaders are also expected to criticise Russia at a summit in Brussels on Thursday, EU officials say.

After years of inaction in Syria, the 28-nation EU is now desperate to stem the flow of migrants. Its stark criticism of Moscow underscores just how far diplomatic efforts have faltered since a U.N. meeting in New York in late September, where Europe and the United States looked to Russia for help.

Russian incursions into Turkish airspace and air strikes directed not at Islamic State militants but at moderate opposition groups have alienated the West, while leaving EU and U.N diplomacy in disarray, diplomats said.

Plans have evaporated for a 'contact group' working with Russia, the United States, Iran and Saudi Arabia to find a post-conflict settlement, while EU diplomats have few ideas about how to find a political solution.

"All Assad's main opponents are dead, in jail or in exile. And nobody wants another Libya," said one EU diplomat involved in the discussions, referring to Libya's collapse after its veteran leader Muammar Gaddafi was ousted.

The EU's own position on Assad remains unclear, with no agreement on whether Assad could play a role in agreeing a ceasefire and paving the way for elections, or whether he should go into exile or into prison immediately.

Spain's foreign minister reiterated Madrid's view that the West will need to negotiate with Assad to stabilise Syria.

"Negotiations are done between enemies," Jose Manuel Garcia-Margallo told reporters.

But France, which is investigating Assad for war crimes, insisted that he must quit as soon as possible.

"For peace in Syria, we need a political transition. That must be done without Assad," said France's European affairs minister Harlem Desir.

Britain says Assad cannot be allowed to remain as president but is willing to discuss how and when he might leave.

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