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GENEVA (Reuters) - The ceasefire in the Syria war is holding for the most part but humanitarian aid is still not getting through to besieged areas where food is running out, the U.N. envoy said on Thursday.

Envoy Staffan de Mistura voiced concern that 23 buses and Syrian drivers used in recent evacuations were being stopped from leaving the villages of Foua and Kefraya in Idlib province by armed groups. He called for them to be allowed to leave.

“These are not U.N. officials, these are Syrian buses with Syrian drivers. And that is not to happen because this complicates then tit-for-tat approaches,” de Mistura told reporters in Geneva after the weekly meeting of the humanitarian task force.

The ceasefire brokered by Russia and Turkey last month was largely holding, he said But fighting was still going on in two villages in the Wadi Barada valley, the site of water pumping facilities serving more than 5 million people in Damascus. Five other villages in the area had reached an agreement with the government, he said.

Water engineers are ready to repair the damaged facility, security permitting, he said, although two attempts to do so had been blocked by armed groups.

“Military activities in that area means also the potential of further damaging water pumps and water supplies,” he said.

De Mistura said he understood that the United Nations would be invited for talks in the Kazakh capital of Astana on Jan. 23, being organised by Russia and Turkey.

That meeting was aimed at deepening the cessation of hostilities and forming “some type of political broad lines,” which could contribute to Geneva peace talks he has convened around Feb. 8, de Mistura said. But there had been no formal invitations or confirmed dates for Astana.