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 independently 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.

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,” he 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.

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.

Nuland said, “We are calling this out now in the hope that we can stop what could be a potential massacre.”

Nuland declined to say if Washington or anyone else in the international community might take proactive measures.

There are no prospects for a NATO intervention like the one that helped topple Libya’s Moammar Gadhafi ― in part because Russia has promised to veto such a plan.

The bloodshed has led to broad condemnation of the regime, although Russia, Iran and China have stood by President Bashar Assad. Russia and China have vetoed two Security Council resolutions that threatened sanctions against Syria.

On Monday, Russia’s Deputy Prime Minister Dmitry Rogozin defended his country’s arms sales to Syria.

“Under no circumstances can the arms supplied to Syria be used against the civilian population,” Rogozin was quoted as saying by the ITAR-Tass news agency.

“Weapons do not shoot all by themselves. It is people who shoot from them,” he said.

Despite Russia’s strong stance, Britain’s Foreign Secretary William Hague said Monday the U.K. will not rule out the use of an international military intervention.

“Each day reports emerge of savage crimes,” Hague told lawmakers at the House of Commons. “These deliberate military tactics are horrifyingly reminiscent of the Balkans in the 1990s.”

He said Britain was focused on diplomatic efforts, but would “not rule out any other option which could at any stage stop the bloodshed.”

The U.S. and its allies have shown little appetite for getting involved in another Arab nation in turmoil. There also is a real concern of a spillover effect for other countries in the region.

In Israel, the deputy military chief warned that Syria’s large chemical weapons stocks could be trained on the Jewish state. According to Maj. Gen. Yair Naveh, Syria has the largest arsenal of chemical weapons in the world. If the Syrians had the chance, he said, they would “treat us the same way they treat their own people.”

Syria has not acknowledged possessing chemical weapons, so the size of its arsenal is not known.