With diplomatic efforts deadlocked, Syria's protracted conflict remains the biggest headache for global powers, and Hague described it as the world's top destination for jihadists.

The rebel al-Nusra Front fighting President Bashar al-Assad pledged allegiance to al Qaeda leader Ayman al-Zawahri on Wednesday, fuelling concerns that Syria could become a hotbed for Islamist militants in the region.

"Moscow is seriously concerned with a bigger interest in Syria on the part of al Qaeda, the observed plans by international terrorists to turn this country into their main springboard in the Middle East," Russia's Foreign Ministry said in a separate statement on Thursday.

Leaders of the Syrian National Coalition were present on the sidelines of the London meeting to ask for more humanitarian help but no promises were made, according to officials.

In their statement, the ministers "called for greater humanitarian assistance and for improved and safe access to the Syrian people by humanitarian agencies in co-ordination with all parties to the conflict".

On Iran, they expressed concern about the lack of progress in the latest talks aimed at resolving a decade-long nuclear dispute that threatens to trigger a new war in the Middle East.

"People were concerned about that, that time is not unlimited," the US official said.

"They didn't feel that the Iranians were bringing anything significant to the table, or anything new."

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