Luxembourg (AFP) - The European Union on Monday strongly condemned Russia for the bombardment of the Syrian city of Aleppo, saying devastating air strikes by Moscow and Damascus could amount to war crimes.

EU foreign ministers meeting in Luxembourg also warned that the 28-nation bloc could impose additional sanctions against Damascus but decided against targeting Russia despite US and British calls to punish Moscow as well.

As if on cue to steal the diplomatic headlines, Moscow announced it would call an eight-hour halt to hostilities on Thursday -- the very day that EU leaders are due to discuss Russia at a summit in Brussels.

EU foreign affairs chief Federica Mogherini said the ceasefire -- announced as the ministers were meeting -- was a "positive step", but might not be long enough to allow aid to reach besieged Aleppo.

The ministers took a harsher tone, issuing a statement condemning Moscow by name for the first time for its role in aiding longtime ally President Bashar al-Assad to pound eastern Aleppo.

"Since the beginning of the offensive by the regime and its allies, notably Russia, the intensity and scale of the aerial bombardment of eastern Aleppo is clearly disproportionate," the statement said.

"The deliberate targeting of hospitals, medical personnel, schools and essential infrastructure, as well as the use of barrel bombs, cluster bombs, and chemical weapons, constitute a catastrophic escalation of the conflict ... and may amount to war crimes."

- Harder line -

The reference to war crimes is potentially significant since, if pressed, a case could be taken to the International Criminal Court in the Hague.

EU diplomatic sources said the reference to Russia by name in the statement was significant, signalling a much harder line compared with earlier phrasing which referred only to Assad's "allies" on the ground.

They said there had been some hard talking to get such wording given opposition from some member states who fear worsening relations with Russia, on which the EU has already imposed sanctions over the Ukraine crisis.

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Such reservations effectively prevented any discussion of possible sanctions against Russia over Syria, they said.

French Foreign Minister Jean-Marc Ayrault said going into the talks that the aim was to "examine all the options to put much stronger pressure on the regime of Bashar al-Assad and his allies."

As the EU statement was coming through, Moscow announced a ceasefire from 0500 GMT to 1300 GMT on Thursday -- coinciding with the opening of an EU leaders summit in Brussels at which ties with Russia are top of the agenda.

The EU ministers said the bloc "firmly believes that there can be no military solution to the conflict" and called directly on Russia to agree a ceasefire to open the way to peace talks.

- 'Surreal' talks -

Mogherini insisted that while the EU had no military role to play, it would do everything possible to support peace efforts to end a conflict which has cost some 300,000 lives and displaced half the population.

This would include talking to regional powers, including Iran, which backs Assad and Turkey, opposed, and all interested parties who could help plan a "post-conflict future" for the country.

Mogherini said it might seem "surreal" to do this now, with the war raging, but doing so would support peace efforts and ensure the EU played a major role in resolving the conflict.

Fresh "brainstorming" talks held Saturday in Lausanne, Switzerland with the main players in Syria's conflict failed to produce any breakthrough.

The EU already has extensive sanctions in place against Syria, including oil and arms embargoes, plus restrictions on more than 200 individuals and 70 entities.