The US has rejected a joint request by Moscow and Damascus to let them evacuate refugees stranded in the Rukban camp within the US-controlled zone in Syria to areas under Damascus' control, the Russian military said.

The first column of buses accompanied by Russian military police set off for the camp on Friday. They were supposed to enter through one of the two mobile checkpoints opened by the Russian and Syrian forces last month and ferry the refugees to other parts of Syria for permanent resettlement.

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However, the planned operation was apparently aborted after the US military, which controls the 55km area around the US military base at Al-Tanf – including the Rukban camp – refused to grant permission for the buses to proceed, Sergey Solomatin, head of the Russian Center for Reconciliation of the Warring Parties in Syria, said on Saturday.

"Despite the successful completion of all preparatory work by the Syrian government to bring the refugees out of the Rukban camp and to the places of permanent resettlement, the US side has rejected the joint call [by Russia and Syria] asking them to let the transport columns into the Al-Tanf zone," Solomatin said.

Over 40,000 refugees, mainly women and children, are estimated to be living in the camp after being displaced by years of conflict. They are said to be suffering from a shortage of adequate medical assistance and basic supplies.

It is not clear why the US military blocked access for the Russia-backed mission. Washington has repeatedly called on Moscow to facilitate the delivery of UN humanitarian aid to the Rukban camp, calling for Damascus to provide "rapid, safe and unhindered" access to the refugees.

"We urge that access for humanitarian assistance must continue to Rukban and to all areas in Syria where people are in need," the US State Department said in February.

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