BEIRUT (Reuters) - The Syrian military bombarded an area northeast of Damascus on Monday, keeping up its attack on a rebel-held district where around 100 people were killed on Sunday in an air strike on a market, the Syrian Observatory for Human Rights said.

Syrian refugee children look out from their tent during a visit by United Nations (U.N.) Humanitarian Chief and Emergency Relief Coordinator Stephen O'Brien to their makeshift settlement in Saadnayel in Lebanon's Bekaa Valley August 17, 2015. REUTERS/Mohamed Azakir

Sunday’s air strike on Douma, 10 miles (15 km) northeast of Damascus, was one of the bloodiest attacks of its type in the four-year-long war that has killed an estimated quarter of a million people and driven 10 million from their homes.

The U.N. envoy for Syria condemned the air strike, while the United States said it showed that President Bashar al-Assad had lost legitimacy and called for a “genuine political transition”. But Syria’s political opposition said the protracted conflict could not be resolved if such attacks went unpunished.

“Any talk of political and peaceful solutions in the shadow of massacres and excusing the criminal from punishment will have no meaning in realizing stability in Syria,” said Khaled Khoja, head of the Turkey-based political opposition coalition.

Sixty bodies were buried on Sunday night in two mass graves, said a spokesman for the Syrian Civil Defense force in Douma, a rescue service operating in rebel-held areas. Another 35 were buried on Monday, and the death toll was over 100, he said.

“It was really difficult to identify the bodies of the martyrs. Some of them were burned to the bone, so we couldn’t add them to the documented list,” said the 28-year-old spokesman, who declined to give his real name for security reasons. His house was destroyed in the bombing, he added.

A Syrian military source said the air force attacks on Sunday on Douma and the nearby area of Harasta targeted an insurgent group, Islam Army, and were a response to recent attacks on nearby government-held areas.

The opposition National Coalition for Syrian Revolutionary and Opposition Forces said the attack aimed to inflict as many civilian casualties as possible. The dead included children.

A United Nations commission of inquiry into the Syrian war has previously accused the government of indiscriminate shelling and aerial bombardment, including the use of barrel bombs regularly dropped on crowded areas.

The Observatory reported that at least 96 people were killed. After rescue workers arrived following an initial attack, the warplanes bombed the area again.

U.N. envoy Staffan de Mistura said in a statement that Sunday’s air strike on Douma was devastating. “Attacks on civilian areas with aerial indiscriminate bombs, such as vacuum bombs, are prohibited under international law,” he said.

“Hitting crowded civilian markets killing almost one hundred of its own citizens by a government is unacceptable in any circumstances.”

De Mistura’s statement said the attack followed “the indiscriminate shelling of Damascus last week by armed opposition groups and the cutting of water supplies, all measures which affect civilians and are also unacceptable”.

CEASEFIRE CRUMBLES

Washington, which has called on they Syrian president to step aside, said Sunday’s strike on Douma was “just another reminder of the inhumane acts perpetrated daily by the Assad regime against the Syrian people.”

“These abhorrent actions underscore that the Assad regime has lost legitimacy and that the international community must do more to enable a genuine political transition,” National Security Council spokesperson Ned Price said.

The air strikes and fighting raging elsewhere across Syria underscore huge challenges facing a new diplomatic effort to advance potential solutions to the conflict.

At a meeting of their two foreign ministers on Monday, Russia and Iran, whose support has been crucial to Assad’s survival, said his future must be decided by Syrians. Moscow also said it opposed any pre-negotiated exit of the Syrian president as part of a peace deal.

A rare ceasefire between the warring sides crumbled at the weekend, when fighting resumed in the town of Zabadani near the Lebanese border and two villages in the northwest.

The war in and around Damascus - Assad’s seat of power - has also escalated. Islam Army, one of the most powerful rebel groups in the eastern Damascus region, attacked government positions in Harasta on Saturday.

Last Wednesday, a barrage of insurgent rockets killed 13 people in Damascus, and government air strikes in the nearby Eastern Ghouta region killed 31 people, the Observatory said.

The Syrian military source said “any escalation towards Damascus” would be met by a “very strong and decisive response”, and denied that civilians were targeted on Sunday.

An insurgent rocket attack on the government-held coastal city of Latakia meanwhile killed at least three people, in the second lethal attack there in four days, the Observatory and state media reported.

State television said the rockets fired at Latakia came from insurgent-held areas to the north of the city. On Thursday, rocket attacks killed at least two people in the city.