UN begins evacuation as Syrian peace talks fray

The UN began evacuating hundreds of people from besieged Syrian towns in a rare sign of humanitarian progress, as the US Air Force for the first time deployed a B-52 bomber against the Islamic State group.

In Geneva, where UN-brokered peace talks have faltered as a fragile cease-fire has come under strain, the Syrian government meanwhile said it was pushing on with indirect negotiations, calling the opposition's freeze on taking part "absurd theater".

The anti-government High Negotiations Committee this week suspended its participation in the talks until it could see progress on political transition and humanitarian issues.

The talks are aimed at putting an end to Syria's five-year war with a political transition, a new Constitution, and fresh elections by September 2017.

But the main block remains the fate of President Bashar Assad, whom the opposition says cannot be a part of a transitional government.

On the ground, the UN secured the evacuation of dozens of people needing medical attention from four besieged towns on Wednesday, as part of a complex humanitarian mission, while the US-led coalition upped the ante on the battlefield.

Washington sent in a B-52 bomber to destroy a weapons storage facility south of Mosul, the Pentagon said.

UN spokesman Stephane Dujarric said plans were under way to evacuate some 500 people including the sick, wounded and their family members "in urgent need of lifesaving medical attention" from four besieged towns.

On the outskirts of the rebel-held town of Madaya, seven large white buses were used to evacuate residents.

The evacuations were taking place from rebel-held Zabadani and Madaya near Damascus, and the government-held towns of Fuaa and Kafraya in northwest Idlib province.