The US has voiced fears that the Syrian government is preparing a new massacre, amid reports that regime troops have renewed attacks on rebel strongholds.
There have been reports of shelling in the city of Homs and the use of mortar bombs, helicopters and tanks on towns near the coast.
Travelling on the road between Homs and Rastan, Sky News' foreign affairs editor Tim Marshall saw families fleeing for their lives and evidence of attacks from the air by the Syrian army on civilian targets.
The reports have prompted Washington to voice its concern that President Bashar al Assad's regime is planning to carry out new atrocities, after the massacre of 55 people last week in the village of al Qubeir and at least 108 near Houla last month.
"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," US State Department spokeswoman Victoria Nuland said.
Ms Nuland said the tactics showed the Assad regime, cracking down on the most severe threat to his family's four-decade rule, was "increasingly desperate".
"What government voluntarily uses helicopters and fires from them on their own civilians if they're not desperate? What government depends on a bunch of thugs in trucks - irregulars - if they're not desperate?" she asked.
But Ms Nuland again ruled out US military intervention.
Meanwhile, Foreign Secretary William Hague has again called for a peaceful transition of power in Syria.
At a news conference in Islamabad, where he is holding talks with Pakistan's foreign minister, Mr Hague stressed the difference between the situation in Syria and that in Libya, where intervention helped to topple the Gaddafi regime.
"This is reminiscent of Bosnia in the early 90s so I don't think we should be thinking of it in the terms of the Libya situation last year," he said.
"All our efforts are going in to supporting a peaceful transition in Syria and a peaceful solution, because any violent solution would clearly involve many more deaths."
Earlier, the UN-Arab League envoy , Mr Annan, said he was "gravely concerned by the latest reports of violence coming out of Syria and the escalation of fighting by both government and opposition forces".
"There are indications that a large number of civilians are trapped in these towns," Mr Annan added.
Two months ago, Mr Annan drew up a ceasefire for the country, but it has not been fully implemented and is being violated daily.
Footage which appears to show the renewed shelling in Homs cannot be independently verified, but Marshall , who was in the city, said it "makes sense".
He said he could hear what he thought to be mortar rounds exploding about every two minutes, along with occasional gunfire.
"We don't know exactly where that video is from, but that is the type of situation that I'm looking at," he said.
He added that the districts being attacked were "absolute hell". He said: "The rest of it is tense, difficult and half-empty because people have fled.
"It's clearly a terrible situation for the people living here - especially those who are on the receiving end of these mortar rounds," he added.
There was also violence in the central province of Hama, activists claimed, as regime troops targeted towns and villages opposed to the Assad regime.
The Syrian Observatory for Human Rights said nationwide violence cost the lives of at least 106 people on Monday, including 77 civilians and 23 Syrian troops.