Syrian government warplanes bombed a market in a rebel-held town outside Damascus on Sunday, killing at least 82 people in one of the bloodiest regime attacks in the country's war.

The series of strikes on the town of Douma overwhelmed makeshift clinics, with bodies lying side-by-side on a bloodied floor as medics struggled to treated waves of wounded.

The head of the Syrian opposition National Coalition, Khaled Khoja, called the attacks as "massacre" and pledged they "will not go unpunished".

The deaths came as the UN's new humanitarian chief visited Syria for the first time since his appointment in May.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said regime warplanes had carried out at least 10 strikes on Douma, most of them hitting a marketplace.

It said that at least 250 people were wounded, with civilians accounting for most of the dead, and the toll could rise further because many of the injured were in a serious condition.

Observatory director Rami Abdel Rahman said locals had gathered after a first strike hit the market to help evacuate the wounded when more attacks came.

At least six raids hit the market, with the others striking nearby in the centre of town, Abdel Rahman said, confirming that the attack was among the bloodiest regime strikes in Syria's conflict.

An AFP photographer in Douma described the attack as the worst he had covered in the town in the opposition bastion of Eastern Ghouta.

In one makeshift clinic, whole sections of floor were covered with rows of the dead, as volunteers worked to wrap each victim in a white shroud.

- Waves of wounded -

Frantic residents brought in the injured, who were treated on chairs, beds and the floor if necessary as the clinic overflowed with patients.

Two young boys sat on a stretcher with blood drying on their faces as they awaited treatment, one resting as though exhausted while the other cried.

Eastern Ghouta is the regular target of government air strikes and has been under siege for nearly two years, with regime forces tightening the blockade since the start of 2015.

Amnesty International earlier in the week accused the government of committing war crimes in Eastern Ghouta, saying its heavy aerial bombardment of the area was compounding the misery created by the blockade.

The group also accused rebels in the area of war crimes for firing rockets indiscriminately at the capital.

Sunday's strikes on Douma came as new United Nations humanitarian chief Stephen O'Brien held talks with government officials in Damascus on his first trip to Syria since being appointed.

O'Brien met Foreign Minister Walid Muallem and expressed a willingness to work with the government to alleviate humanitarian suffering, state media said.

Close to 12 million people have been uprooted by Syria's conflict, with more than four million becoming refugees and another 7.6 million internally displaced.

On Saturday, O'Brien met the deputy foreign minister and visited the central city of Homs, which is now mostly under government control.

- 'Syria needs peace' -

"Beyond destruction of buildings lies destruction of lives. Syria needs peace," he wrote on Twitter.

"We are committed to continuing to support humanitarian efforts in Syria. Equal access to all people in need (is) vital for our work," O'Brien added.

Elsewhere, the Observatory said at least 27 opposition fighters and 15 government forces had been killed in battles since Saturday around the village of Tasneen, in central Homs province.

The fighting there began after a truce involving two government-held villages in Idlib province and a rebel bastion near Damascus broke down.

The truce for the villages of Fuaa and Kafraya and the town of Zabadani crumbled early Saturday after negotiators failed to reach a broader deal.

Fighting in all three areas continued on Sunday, with rebels firing missiles into Fuaa and Kafraya and government and allied forces battling the opposition in Zabadani.

Also Sunday, a US-trained rebel group said that Al-Qaeda affiliate Al-Nusra Front had freed seven of its members kidnapped two weeks earlier.

The group, known as Division 30, is among the units participating in a US-led programme in Turkey to train forces to fight the Islamic State group.

But after the first 54 members of the force entered Syria in July, Al-Nusra kidnapped 13 of them, including a commander, and at least three more were killed in clashes with the jihadist group.

Al-Nusra accused the force of serving US interests and paraded several of the captives in a video.