BEIRUT – 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.

Additional Photos

This image taken from amateur video shows explosions rocking the central Syrian city of Homs on Monday. The Associated Press

State Department spokeswoman Victoria Nuland expressed concern about reports the regime “may be organizing another massacre” in Latakia province on the Mediterranean coast, 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.

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 on 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, Abdul-Rahman said. Another three men and two women were killed while trying to flee, he said.

In the nearby village of Laj, another 11 slain men lay in a room, their names scrawled on papers tucked into their clothes, according to amateur video.

A car bomb exploded in the city of Deir al-Zour, killing 10 people, Abdul-Rahman said.

“What we are seeing right now are fierce clashes as the Syrian army tries to take back positions held by the rebels,” Abdul-Rahman said. “There are many deaths in the rebel ranks.”

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.

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.

Syrian activists say 13,000 people have been killed since the uprising began in March 2011. The situation has grown increasingly chaotic in recent months, and it is difficult to assign blame for much of the bloodshed.

The government restricts journalists from moving freely, making it nearly impossible to independently verify accounts from either side.

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