BEIRUT (AP) - The United States accused the Syrian government of using "new horrific tactics" Monday, as U.N. observers reported Syrian helicopters were firing on rebellious areas and concerns mounted that civilians were trapped in besieged cities.
Violence in Syria has spiked in recent weeks, as both sides ignore an internationally brokered cease-fire that was supposed to go into effect April 12 but never took hold.
State Department spokeswoman Victoria Nuland expressed concern about reports the regime "may be organizing another massacre" in Latakia province, where U.N. monitors have been impeded.
Speaking to reporters in Washington, Nuland warned, "People will be held accountable."
Activists reported more than 50 people killed across Syria on Monday, with clashes between military forces and rebel fighters in Homs, Idlib and Latakia provinces. The death toll and the online videos were impossible to verify.
According to videos posted online, fireballs of orange flame and black rubble exploded in the air as waves of shells pounded residential buildings in the central city of Homs Monday. The sounds of shells whooshed through the sky amid sporadic machine gun fire.
Syrian soldiers chased down and killed rebels who set fire to one of their tanks in a farming area close to the Orontes river in the Idlib province, said Rami Abdul-Rahman of the Britain-based activist group Syrian Observatory for Human Rights, which uses a network of sources on the ground.
The attack killed seven soldiers and a civilian, he said. There was no confirmation from state media.
Another three men and two women were killed while trying to flee, Abdul-Rahman said.
International envoy Kofi Annan said Monday he was "gravely concerned" about the escalation of fighting in Syria, citing the shelling of opposition areas in central Homs province and reports of mortar, helicopter and tank attacks in the town of Haffa and its surrounding villages in Latakia province on the Mediterranean coast.
Annan demands both sides "take all steps to ensure that civilians are not harmed," said his spokesman, Ahmad Fawzi. "There are indications that a large number of civilians are trapped in these towns," the statement said.