DAMASCUS (AFP) - 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 series of raids on the town of Douma, in the rebel bastion of Eastern Ghouta, was one of 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".

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

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

Observatory head Rami Abdel Rahman said government aircraft carried out another four air strikes on Douma on Monday morning, but he had no immediate details on casualties.

An AFP photographer said residents were trying to bury victims of Sunday's attack at the local cemetery, 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. He saw dozens of bodies lined up on the bloodied floors of one of Douma's makeshift clinics, as medics struggled to treat waves of wounded.

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

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

Amnesty International last week accused the government of committing war crimes there, saying its heavy aerial 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.

In Geneva, UN envoy Staffan de Mistura called the Douma attacks "devastating".

"Hitting crowded civilian markets killing almost one hundred of its own citizens by a government is unacceptable in any circumstances," he added.

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

Earlier Monday, the UN's humanitarian chief Stephen O'Brien, on his first trip to Syria since taking the post in May, also condemned the attack.

At a news conference in Damascus, he said 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.

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.

In a statement, 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," the statement read.

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

"The UN Security Council and the international community's lukewarm response is a contributing factor in the escalation of massacres against Syrian civilians," the Coalition said.

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.

The Observatory confirmed the attack in Latakia city but said three had been killed.

Meanwhile, Russian Foreign Minister Sergei Lavrov, meeting his Iranian counterpart Mohammad Javad Zarif, another Syria ally, insisted Moscow did not accept Assad's departure as a prerequisite for peace.

"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.

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