The death toll in Syrian government air strikes on a rebel-held town outside Damascus neared 100 on Monday, with UN officials expressing horror at the "unacceptable" attack.

Sunday's raids on Douma, in the rebel bastion of Eastern Ghouta, were among the bloodiest regime attacks in Syria's four-year war.

They came almost exactly two years after devastating chemical weapons attacks on the same region that much of the international community blamed on the Syrian government.

The United Nations' Syria envoy Staffan de Mistura described the attacks as "unacceptable in any circumstances".

The United States also condemned the "brutal" strikes which State Department spokesman John Kirby said shows "the regime's disregard for human life".

Syria's main opposition body in exile, the National Coalition, denounced the strikes and the international community's "lukewarm response" towards the war's civilian casualties.

At least 96 people were killed and 240 wounded in 10 air strikes on a marketplace and other parts of Douma, said the Syrian Observatory for Human Rights, a Britain-based monitor.

Observatory head Rami Abdel Rahman said government aircraft carried out another four strikes on Monday morning, without providing a casualty toll.

An AFP photographer said residents were trying to bury victims of Sunday's attack, despite the renewed strikes.

He said the number of dead in repeated raids on Douma had forced gravediggers to create a mass grave at least four layers deep to accommodate the dead.

The photographer described Sunday's attack as the worst he had covered in the town.

- Dozens of bodies -

He saw dozens of bodies lined up on the bloodied floors of a makeshift clinic, as medics struggled to treat waves of wounded.

Eastern Ghouta, a rebel bastion regularly targeted by government air strikes, has been under a suffocating siege for nearly two years.

Amnesty International has accused the government of committing war crimes there, saying its heavy bombardment of the area was compounding the misery created by the blockade.

It also accused rebels in the area of committing war crimes by firing rockets indiscriminately at Damascus.

De Mistura called the Douma attacks "devastating".

"Hitting crowded civilian markets killing almost 100 of its own citizens by a government is unacceptable in any circumstances," he said in Geneva.

He said the deaths underscored that there was no military solution to the conflict.

UN humanitarian chief Stephen O'Brien, on his first trip to Syria since taking the post in May, also condemned the attack on Monday, telling a news conference in Damascus, he was "horrified by the total disrespect for civilian life in this conflict".

"I am particularly appalled by reports of air strikes yesterday causing scores of civilian deaths and hundreds injured right in the centre of Douma, a besieged area of Damascus," O'Brien said.

The European Union also condemned "escalating" violence in Syria.

Syria "has already become the world's largest humanitarian crisis", the EU said, urging all warring parties to respect international humanitarian law and violators to be held accountable.

- 'Deliberate' targeting of civilians -

At least 240,000 people have been killed in Syria's war, which began in March 2011 with protests against President Bashar al-Assad's regime.

The opposition National Coalition accused the government of "deliberately" targeting civilians in Douma.

"Assad's jet fighters fired missiles on marketplaces at (a) busy time when they are densely crowded with the intention of inflicting as many civilian casualties as possible," a statement read.

It also said the international community's failure to respond to such atrocities contributed to the violence.

Coalition head Khaled Khoja said the Assad regime's "boldness in committing massacres against civilians for 53 consecutive months depends on international silence that amounts to complicity".

Elsewhere, rebel fire on the provincial capital of Assad's coastal heartland Latakia killed six people and wounded 19 on Monday, state TV said. According to the Observatory there were only three deaths.

Russian Foreign Minister Sergei Lavrov said Moscow did not accept Assad's departure as a prerequisite for peace, at a meeting with his Iranian counterpart Mohammad Javad Zarif.

"While some of our partners believe that it is necessary to agree in advance that at the end of the transitional period the president will leave his post, this position is unacceptable for Russia," Lavrov said, without naming Assad.

Lavrov last week hosted Saudi Arabia's foreign minister and representatives of the Syrian opposition, who all insisted Assad must go.

In Washington, State Department spokesman Kirby said "Assad has no legitimacy to lead the Syrian people".