United Nations observers investigating reports of a massacre of some 200 people visited the village of Tremseh in western Syria on Sunday for the second time and concluded that the Syrian government had targeted opposition supporters, appearing to contradict claims by some opposition groups that the Syrian military was targeting only civilians.

“The attack appears targeted at army defectors and activists,” said a statement from the U.N. observers, who based their conclusions on interviews with witnesses and observations of damaged buildings. The U.N. team did not confirm the number of people who may have been killed.

The team confirmed the use of artillery, mortars and small arms in the attack. U.N. officials said they saw more than 50 houses that were burned or destroyed, some of them with grim signs of the deaths that had occurred inside. “Pools of blood and brain matter were observed in a number of homes,” the observer team said in a statement.

At a news conference in Damascus on Sunday, Jihad Makdissi, a spokesman for the Syrian Foreign Ministry, forcefully denied the use of any heavy weapons.

“Government forces did not use planes, or helicopters, or tanks or artillery,” Makdissi said. “The heaviest weapon used was an RPG,” or rocket-propelled grenade.

The official Syrian Arab News Agency has blamed “terrorists” for the assault. In his remarks, Makdissi claimed that the attack had killed 37 fighters and two civilians, figures much lower than the opposition estimates of casualties.

The International Committee of the Red Cross said Sunday that the conflict is an all-out civil war, a legal status that means the Syrian military and any associated militia groups, as well as opposition fighters, can be prosecuted for war crimes under international humanitarian law.

Damascus, the capital, has been relatively calm throughout the 16-month-long uprising. But heavy fighting broke out Sunday in neighborhoods across the city, and opposition groups said government forces were shelling residential areas.

One video circulated by the Local Coordination Committees, an opposition group, showed thick black smoke rising from Tadamon, a neighborhood in southern Damascus.

Abu Hussam, a Damascus resident who asked that his full name not be used because he feared for his safety, said Sunday’s clashes between the Syrian military and armed rebels were the heaviest fighting he had seen in the capital. “The sound of bullets doesn’t stop,” he said by Facebook chat. Abu Hussam, who evacuated his family to a safer neighborhood, said many people were taking shelter in mosques.

After visiting Tremseh on Sunday, the U.N. observer team released a short video of what it had discovered. The video shows several burned-out buildings pockmarked with shrapnel and bullet holes as well as a mosque with a blasted minaret.

At one point, the video zooms in on a sign that reads “Tremseh School.” A man, apparently inside the school, shows the camera a bloody mattress and pillow as well as bloodstains sprayed across walls and doors. Another burned-out building is identified as a hospital by an off-camera voice. Near the end of the video, two women are shown grieving in a makeshift cemetery, one of them dumping handfuls of dirt on her head.

The U.N. report says that, according to those interviewed, “the army was conducting house to house searches asking for men and their ID cards” and that many of those men were subsequently killed.

Abu Issam, an army defector who helps the rebel Free Syrian Army with recruiting in the area, said the military operation was much more haphazard than that. Reached by telephone, he said that only about 30 rebel fighters were in Tremseh on Thursday, when the attack occurred, and that many civilians were killed, including two of his sons.

Abu Issam said his 16-year-old son was wounded by a piece of shrapnel during a round of shelling early Thursday. He was taken to a local hospital by his brother and a number of friends. As he was being treated, the hospital was attacked by pro-government militiamen known as the shabiha, the man said.

“They killed everyone inside,” Abu Issam said. He said he has little hope that the U.N. observer team will dig up the facts of what happened in Tremseh. “How long will the U.N. keep giving time for the regime?” he asked.

Suzan Haidamous and a special correspondent in Beirut contributed to this report.