The European Union's counter-terrorism co-ordinator has warned that hundreds of Europeans are now fighting with rebel forces in Syria against Bashar al-Assad's regime.

Gilles de Kerchove estimated the number in Syria at about 500 - including fighters from Ireland, the United Kingdom and France.

Intelligence agencies are concerned some could join groups linked to al-Qaeda and later return to Europe to launch terrorist attacks.

Yesterday, US Secretary of State John Kerry said he and Russian Foreign Minister Sergei Lavrov had agreed to look for ways to revive a Syrian peace plan, but admitted that doing so would be extremely difficult.

Mr Kerry also backed away from earlier comments suggesting he was calling for increased NATO contingency planning on Syria.

He said that he and Mr Lavrov had discussed ways to revive a peace plan agreed in Geneva last June that called for a transitional government.

"We are both going to go back, we are going to explore those possibilities, and we are going to talk again about if any of those other avenues could conceivably be pursued," Mr Kerry said.

He said that while there might be a difference of opinion between Russia and the United States about when and how Mr Assad might leave office, "I don't think there's a difference of opinion that his leaving may either be inevitable or necessary to be able to have a solution".