Free Syrian Army members, with covered faces and holding weapons, sit by the side of a street in Qaboun district, Damascus

REUTERS

United Nations monitors said Syrian helicopters fired on rebel strongholds north of Homs on Monday and said many women and children were reported trapped in the city, calling for "immediate and unfettered access" to the conflict zones, reports Reuters.

International mediator, Kofi Annan also expressed grave concern about violence in Homs and in Haffeh, a mainly Sunni Muslim town near the Mediterranean coast, where the U.S. State Department said it feared a "potential massacre".

The U.N. observers, tasked with monitoring Annan's April ceasefire deal which failed to stem the violence in Syria, have instead been cataloguing mass killings, bombings and clashes in which many hundreds of Syrians have died.

The outside world, divided in its approach towards President Bashar al-Assad's crackdown on a 15-month-old uprising, has been unable to halt the violence despite broad international support for Annan's tattered peace plan.

"UN observers reported heavy fighting in Rastan and Talbiseh, north of (Homs), with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms," U.N. spokeswoman, Sausan Ghosheh said in a statement.

It was the first time the U.N. monitors have verified repeated allegations by activists that Assad's forces have fired from helicopters in the military crackdown on rebels.

The observers "also received reports of a large number of civilians, including women and children trapped inside (Homs) and are trying to mediate their evacuation," Ghosheh said.

U.N. observers reported that Free Syrian Army rebels captured army soldiers, she added, calling on "all sides to stop the killing and human rights abuses to ensure the protection of civilians and to respect international law."

The Syrian Observatory for Human Rights said 50 civilians were killed across Syria on Monday, nearly half of them in the northern province of Idlib. Twenty-one soldiers and security forces were killed, most of them in rebel bomb attacks, it said.

Syria's state news agency reported military funerals on Monday for 26 people "targeted by armed terrorist groups as they carried out their national duty".

A spokesman for Annan said he was gravely concerned by the latest reports of violence and "the escalation of fighting by both government and opposition forces".

Annan expressed particular concern at recent shelling in Homs, where activists said on Sunday government forces killed 35 people in one of the biggest bombardments since his ceasefire deal was supposed to come into effect on April 12.

"(Annan) is particularly worried about the recent shelling in Homs as well as reports of the use of mortars, helicopters and tanks in the town of Haffeh," his spokesman, Ahmad Fawzi said in a statement.

"There are indications that a large number of civilians are trapped in these towns," Fawzi said, adding that Annan "demands that the parties take all steps to ensure that civilians are not harmed, and further demands that entry of the U.N. military observers be allowed to the town of Haffeh immediately."