U.N. monitors say the Syrian government is using helicopters for air attacks against rebel strongholds, and there are fears that many civilians are trapped in besieged cities.

The United States and the U.N. both voiced deep concern about the latest violence in Syria.

U.N. and Arab League envoy Kofi Annan says he is “gravely concerned” about the violence in Homs and Haffeh, where rebels have been attacked by artillery, helicopters, tanks and mortars. fighting includes shelling, helicopter attacks, tanks and mortar fire. Monday was the first time the U.N. has said it could verify activists' reports that Syrian military helicopters were attacking the rebels.

Mr. Annan's spokesman says there are “indications a large number of civilians are trapped in those towns.”

At the U.S. State Department, spokeswoman Victoria Nuland said there is “deep alarm [about] reports from inside Syria that the regime may be organizing another massacre” of civilians in at least six places around Syria.

“Syrian forces have already escalated their oppression in direct defiance of their commitments to the Annan plan by using now new, horrific tactics, including the firing on civilians from the air by helicopters. Added to the use of irregular forces, the shabiha, this constitutes a very serious escalation.”

Violence across the country Monday claimed more than 60 lives, according to activists' reports, but it has been impossible to confirm the toll independently. There are reports of an aerial assault by government forces targeting the strategic river city of Rastan, which has resisted repeated government offensives for months.

Rami Abdul-Rahman of the Britain-based Syrian Observatory for Human Rights said security forces began increasingly relying on helicopter attacks after government troops suffered major losses in recent battles.