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 U.N. 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 village, checking residents’ IDs and then killing some of them and taking others away.

According to the U.N., the attack appeared targeted at army defectors and activists.

“Pools of blood and brain matter were observed in a number of homes,” a U.N. statement said.

On Sunday, Syria denied U.N. claims that government forces had used heavy weapons such as tanks, artillery and helicopters during the attack Thursday in Tremseh.

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