Syria's 16-month bloodbath crossed an important symbolic threshold Sunday as the international Red Cross formally declared the conflict a civil war, a status with implications for potential war crimes prosecutions.

The Red Cross statement came as United Nations observers gathered new details on what happened in a village where dozens were reported killed in a regime assault. After a second visit to Tremseh on Sunday, the team said Syrian troops went door-to-door in the small farming community, checking residents' IDs and then killing some and taking others away.

According to the UN, the attack appeared to target army defectors and activists.

"Pools of blood and brain matter were observed in a number of homes," a UN statement said.

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An image from amateur video from the group Hama Revolution 2011 purports to show bodies of people killed by violence in Tremseh, Syria, that anti-regime activists blame on the government. (Hama Revolution/Associated Press)

Syria denied UN claims that government forces had used heavy weapons such as tanks, artillery and helicopters during the attack Thursday. If such munitions were directed against civilians, it would constitute a war crime.

Syrian Foreign Ministry spokesman Jihad Makdissi said the violence was not a massacre — as activists and many foreign leaders have alleged — but a military operation targeting armed fighters who had taken control of the village.

"What happened wasn't an attack on civilians," Makdissi told reporters Sunday in Damascus. He said 37 gunmen and two civilians were killed — a far lower death toll than the one put forward by anti-regime activists, some of whom estimated the dead at more than 100.

"What has been said about the use of heavy weapons is baseless," Makdissi added.

UN observers saw attack

The UN has implicated President Bashar Assad's forces in the assault. The head of the UN observer mission said Friday that monitors stationed near Tremseh saw the army using heavy weaponry and attack helicopters.

The fighting was some of the latest in the uprising against Assad, which activists say has killed more than 17,000 people. Violence continued Sunday, with more clashes reported around the capital, Damascus.

The bloodshed appeared to be escalating. On Sunday, the International Committee of the Red Cross said it now considers the Syrian conflict a civil war, meaning international humanitarian law applies throughout the country.

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Tremseh is northwest of Homs, in western Syria.

Also known as the rules of war, humanitarian law grants all parties in a conflict the right to use appropriate force to achieve their aims. The Geneva-based group's assessment is an important reference for determining how much and what type of force can be used, and it can form the basis for war crimes prosecutions, especially if civilians are attacked or detained enemies are abused or killed.

"We are now talking about a non-international armed conflict in the country," ICRC spokesman Hicham Hassan said.

War crimes prosecutions would have been possible even without the Red Cross statement. But Sunday's pronouncement adds weight to any prosecution argument that Syria is in a state of war — a prerequisite for a war crimes case.

When the Red Cross says something "it's always very persuasive," said Louise Doswald-Beck, a professor of international law at the Geneva Graduate Institute. In legal terms, that means a court would be unlikely to decide differently.

But as an internal conflict officially becomes a civil war, the security environment shifts from regular law enforcement to a situation in which international law permits the government to attack rebel fighters, Doswald-Beck said.

"That's why this whole business of Tremseh is interesting," she said.

Mass grave

UN observers first entered Tremseh, a farming-region community of 6,000 to 10,000 people northwest of the city of Hama, on Saturday. They found pools of blood in homes, along with spent bullets, mortars and artillery shells. The evidence added to the emerging picture of what anti-regime activists have called one of the deadliest events of the uprising.

Dozens of bodies have already been buried in a mass grave or burned beyond recognition, and activists were struggling to determine the number of people killed. Estimates range from 100 to more than 150 dead.

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In an image provided by Shaam News Network but not verifiable, a Syrian man on Saturday mourns over the body of a man killed in Homs, in the country's west. Syria denies UN assertions that government forces used heavy weapons during a military operation that left scores dead in Tremseh, about 50 km away. (Shaam News Network/Associated Press)

Activists expect those figures to rise since hundreds of residents remain unaccounted for. Locals believe some bodies are still in nearby fields and others were probably dumped in the river.

Some of the evidence suggested that, rather than the outright shelling of civilians depicted by the opposition, the violence in Tremseh may have been a lopsided fight between the army pursuing the opposition and activists and locals trying to defend the village. Nearly all of the dead are men, including dozens of armed rebels.

Independent verification of the events is nearly impossible in Syria, one of the Middle East's strictest police states, which bars most media from working independently within its borders. The UN observers are in the country as part of a faltering peace plan by UN and Arab League special envoy Kofi Annan.