The United States fears Bashar al-Assad is preparing to carry out a fresh massacre in an opposition stronghold as his regime unleashed helicopter gunships on rebel cities across Syria.
Mourners carry the coffins in Daraa on Saturday
US State Department spokesman Victoria Nuland voiced "deep alarm" for the fate of Haffa, scene of some of the fiercest fighting yet seen in the conflict, amid reports that government tanks and heavy artillery had surrounded the town.
Kofi Annan, author of the now collapsed United Nations-sponsored peace deal, had earlier told how helicopters had strafed rebel positions in Haffa following days of heavy fighting there.
"The United States joins joint special envoy Kofi Annan in expressing deep alarm by reports from inside Syria that the regime may be organiSing another massacre," Miss Nuland said.
"We remind Syrian commanders of one of the lessons from Bosnia: The international community can and does learn what units were responsible for crimes against humanity and you will be held responsible for your actions."
Syria has already experienced two major massacres recently, with 55 people killed last week in al-Qubeir and at least 108, about half of them children, killed in Houla on May 25.
In Haffa, more than 20 soldiers were reported to have been killed by rebels on a single day last week, and 58 altogether in recent days. Video posted online showed the shrouded bodies of ten children aged up to 13 who were killed in a bombardment on Saturday. The footage showed their mothers weeping over them.
Opposition activists in the town of Rastan, which has successfully resisted a government siege for a month, also came under close fire from three gunships yesterday, followed by a heavy bombardment from artillery shells.
Meanwhile the city Homs, at the heart of the rebellion against the Assad regime, was bombarded by concentrated artillery fire as government forces moved to crush the increasingly confident resistance movement.
Last night, Mr Annan warned that a "large number" of innocent civilians were now trapped by the violence and that he was "gravely concerned" by the "escalation of fighting" between the two sides.
"There are indications that a large number of civilians are trapped in these towns," his spokesman, Ahmed Fawzi, said. "The special envoy demands that the parties take all steps to ensure that civilians are not harmed, and further demands that entry of the UN Military Observers be allowed to the town of Haffa immediately."
In a House of Commons statement on the crisis yesterday, William Hague, the Foreign Secretary, accused the Syrian regime of carrying out "savage" and "grotesque" crimes against its people. He said the behaviour of the Assad regime was "morally reprehensible" and that the Syrian people had endured "fifteen months of fear and suffering".
The Foreign Secretary told MPs that 87,000 people had fled to neighbouring countries, 15,000 had been killed and thousands of political prisoners imprisoned during the uprising.
"Each day reports emerge of savage crimes," he said. "The Syrian military are surrounding and bombarding towns with heavy weaponry, and then unleashing militia groups to terrorise and murder civilians in their homes. These deliberate military tactics are horrifyingly reminiscent of the Balkans in the 1990s."
Mr Hague said that Britain was training activists who were monitoring and recording atrocities, including that in Houla last month in which 108 men, women and children were killed.
He also said there was evidence that groups linked to Al-Qaeda had committed acts of violence to "exacerbate the situation".
"We will not rule out any other option which could at any stage stop the bloodshed," he added.
The level of violence in Syria has now returned to that seen before the announcement of a ceasefire in April. Yesterday (Mon), the Syrian Network for Human Rights reported 58 deaths across the country, the majority in Idlib province in the north where the rebel-held town of Maarat al-Numan has been under regime attack for several days.
The town of Rastan, which has not figured prominently in news reporting, has just managed to hold out against government forces. Yesterday however, an activist, Walid Mohammed Abeid, told The Daily Telegraph that its situation was now desperate. "All our houses are destroyed by the bombing, from the air and heavy guns and cannon," he said. "We ask everyone outside to look in their hearts and help us, please, please, please. We are being killed every day."
A spokesman said that in Qusair, between Homs and the Lebanese border, government snipers had shot dead a priest, named as Atallah Ibrahim Bitar.
Fr Atallah had donned his clerical robes while taking food to people hiding from a week-long regime bombardment of the town, much of which is in rebel hands, thinking that he would be protected.
Qusair has a significant Christian minority, and the town's rebel council was swift to deny a report by a Vatican news agency that Christians had been ordered to leave by Islamist elements in the rebel Free Syrian Army.
Most of the town, of both religions, had already fled the fighting in any case, it said in a statement.
In another scene strongly indicative of an outright civil war, rebels captured a battery of missiles near the city of Hama and claimed to have briefly targeted them on the presidential palace in Damascus. They were then forced to flee by an intense counter-attack.