Syrian tanks have fanned out around towns and villages near the Turkish border, widening a crackdown on 12 weeks of anti-government protests.
Damascus said its forces were pursuing rebels through the countryside around Jisr al-Shughour, after consolidating control over the northern town.
Witnesses say troops are pursuing a "scorched earth" strategy, destroying houses and crops as residents flee.
The US, meanwhile, renewed calls on Syria to halt its crackdown.
Damascus insists it is tackling armed groups which it says have been targeting security forces and looting villages.
But some of the several thousands of Syrians who have fled into Turkey say army "thugs" and pro-government mercenaries have gone on the rampage, torching deserted homes and stealing what they can.
'Steal it, or burn it'
Refugees and activists still inside Syria said troops and tanks were cutting off and attacking villages to the east and north of Jisr al-Shughour, leaving a trail of devastation in their wake.
One man who said he had fled Jisr al-Shughour told BBC Arabic: "The army and thugs terrorised the people and frightened the civilians. At the moment there is looting.
"When they find deserted homes, they knock down the doors or gates, enter the houses and steal what they find. If they can't steal it, they burn it."
Tanks were also reported to be entering the town of Maarat al-Numan, to the south-east, where the state media had reported attacks on government buildings and security headquarters in recent days.
One resident in the town told Reuters news agency that helicopters had been ferrying troops near the town.
"We met the [provincial] governor today and he assured us that the army will go in only to arrest 360 people it has on a list," said the man, named as Othman al-Bedeiwi.
"The people of Maarat, however, are sceptical. My name is on the list to be arrested as being a gunman. I never carried a weapon in my life."
As demonstrations against the regime continue to break out, often at night, in many parts of the country, activists report that several parts of the coastal city of Latakia have been sealed off by troops.
Soldiers are also reported to have been deployed at Deir al-Zour, near Iraq's border in the east, and in some suburbs of Damascus, where protests continue.
Condemning the crackdown, White House spokesman Jay Carney said: "President [Bashar al-]Assad needs to engage in political dialogue. A transition needs to take place. If President Assad does not lead that transition, then he should step aside," he told reporters.
On Sunday, troops were confronted by other soldiers trying to defend people in Jisr al-Shughour, according to Syrian refugees who have crossed into Turkey.
Elements from one tank division had even taken up positions by bridges leading into the town in a bid to defend it, they said.
The government says it has been trying to restore order after 120 security personnel were killed in Jisr al-Shughour last week. Residents say they died after a mutiny and fighting between security forces.
Syria has prevented foreign journalists, including those from the BBC, from entering the country, making it difficult to independently verify reports from there.
Hundreds of Syrians have been massing on the border with Turkey - which has already taken in thousands of refugees - preparing to cross over if the army advances.
They are frightened, short of food and sleeping without proper shelter, but do not want to return home until the Assad regime falls, says the BBC's Jonathan Head on the border.
In central Syria, people told BBC Arabic that a 2,000-strong protest organised by lawyers' and engineers' associations took place in the city of Hama.
It was the first such protest called for by the professional groups since the beginning of the protests in Syria.
According to sources close to the Arab League, some member states have introduced a draft resolution to suspend Syria's participation in meetings of the group's council, in protest against the crackdown.
Protests against Mr Assad, who succeeded his father Hafez in 2000, began in mid-March.
Human rights groups say at least 1,300 people have been killed in the crackdown.