ZABADANI, Syria — 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 ceasefire 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 theatre".

The anti-government High Negotiations Committee this week suspended its participation in the talks, which do not include either IS or al-Qaeda’s Syrian offshoot Al-Nusra Front, until it could see progress on political transition and humanitarian issues.

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, days after Defense Secretary Ash Carter announced extra US troops, cash and equipment for the anti-IS campaign in Iraq.

In other signs of increasing tempo, US commandos working with Kurdish troops conducted a raid targeting a senior IS group figure.

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 life-saving medical attention" from four besieged towns.

On the outskirts of the rebel-held town of Madaya, where dozens of residents starved to death last year, seven large white buses were used to evacuate residents.

Dozens of men, women and children got out of the bus and were checked by security officials before boarding once more.

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.

Sieges continue

Aid deliveries to the four localities always occur simultaneously and in equal amounts, and a similar stipulation applied for those being evacuated.

More than four million in Syria live in besieged or hard-to-reach areas, with limited or no access to food or medical supplies.

The UN has long pressed Damascus to grant unrestricted access to these areas, and has asked all sides to end sieges.

The dire humanitarian situation has played a major role in the opposition walking away from the troubled peace talks in Geneva.

An already-shaky ceasefire between the government and non-jihadist rebels was severely strained on Tuesday after at least 44 people were killed in air strikes on two markets in northwest Syria.

The deaths of 37 civilians in a market in the city of Maaret al-Numan and another seven killed in nearby Kafranbel were some of the deadliest attacks since the truce took hold on February 27.

On Tuesday, HNC coordinator Riad Hijab said the opposition delegation could not remain in Switzerland while Syrians were dying on a daily basis. — AFP