BEIRUT: Syrian airstrikes on a Damascus suburb killed more than 80 people Sunday in one of the deadliest such raids of the 4-year-old civil war, as fighting escalated in and around President Bashar Assad’s seat of power at a time when his overstretched forces have been losing ground elsewhere in the country. The head of the Syrian opposition National Coalition, Khaled Khoja, called the attacks a “massacre,” vowing they “will not go unpunished.”

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

The air raids struck the main market in Douma during rush hour, when hundreds of people were out shopping on the first working day of the week in Syria, activists said. The strikes appeared to have been launched in retaliation for the capture of an army base in a nearby suburb a day earlier by the Islamist Army rebel group, which enjoys strong support in Douma.

“This is an official massacre that was carried out deliberately,” said Rami Abdel-Rahman, who heads the Britain-based Syrian Observatory for Human Rights. He said warplanes fired a missile at the market and then launched another after people rushed in to retrieve the wounded.

Abdel-Rahman said a total of four missiles were fired on the market, killing 82 people and wounding over 200. He said the death toll was expected to rise because many of the wounded were in critical condition.

The Local Coordination Committees, another activist group, said the air raids killed at least 100 and wounded about 300, adding that rescue workers were digging through the rubble in search of survivors. Discrepancies in death tolls immediately after an attack are common in Syria.

“The situation is catastrophic,” a Douma-based activist who goes by the name of Mazen al-Shami told the Associated Press via Skype. He said clinics in the area were full and many of the wounded were being rushed in civilian cars to other medical facilities since ambulances are overwhelmed. Shami said mosque loudspeakers issued calls for residents to donate blood.

An amateur video posted online by activists showed some 40 bodies of men and boys lined up on the side of a street as more bodies were being brought in. Another video showed people helping the wounded leave the heavily damaged market area.

Shami said “the regime is targeting civilians to avenge” the capture of the army base, known as the “technical academy,” in the nearby suburb of Harasta Saturday.

Douma is one of the largest suburbs of Damascus and has been held by rebels since the early days of Syria’s conflict.

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 strikes on Douma came as U.N. humanitarian chief Stephen O’Brien held talks with regime officials in Damascus on his first trip to Syria since being appointed.

O’Brien met Foreign Minister Walid al-Moallem 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 the country’s conflict, with more than 4 million becoming refugees and another 7.6 million internally displaced.

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

“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.

The fighting in and around Damascus has escalated in recent days. Last week rebels shelled the capital ahead of a visit by Iran’s foreign minister, Mohammad Javad Zarif, killing more than 30 people.

Iran is preparing a peace initiative that would include a cease-fire and a power-sharing government. Assad would remain in the picture, pending internationally supervised elections, according to a Lebanese politician familiar with the proposal, who spoke on condition of anonymity because he was not authorized to divulge details of the plan.

Zarif will meet his Russian counterpart Sergey Lavrov Monday in Russia to discuss the conflict in Syria and the deal on Tehran’s nuclear program.

Meanwhile, Al-Qaeda’s Syrian affiliate the Nusra Front has freed several members of a U.S.-trained rebel force that it kidnapped two weeks ago, the Western-backed unit said in a statement.

In a statement circulating on social media, Division 30 said seven of its members had been freed by Nusra after being captured in late July.

“We welcome this noble initiative and urge the brothers of the Nusra Front and hope that they will release in the coming hours the group’s commander and other fighters,” the statement said.