Syria says its forces are pursuing rebels through the countryside around Jisr al-Shughour, after consolidating control over the northern town.
Witnesses say the military is pursuing a "scorched earth" strategy in the area, destroying houses and crops.
Activists say many villagers have been detained and at least 10 shot dead by army snipers.
Hundreds of Syrians have been massing on the border with Turkey, preparing to cross over if the army advances.
Turkey has already taken in thousands of refugees.
Syrian military sources have told the BBC that "armed men" who fled from Jisr al-Shughour to the town of Maarat al-Numan would be dealt with in a military operation in the next few days.
Protests against President Bashar al-Assad, who succeeded his father Hafez in 2000, began in mid-March and have spread across the country.
Human rights groups say at least 1,300 people have been killed in the crackdown.
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.
But one report quoted witnesses as saying that hundreds of men aged between 18 and 40 had been detained, following a pattern of previous crackdowns by the Syrian security forces.
"Jisr [al-Shughour] is finished, it is razed," a man who gave his name as Abu Ali told the Associated Press news agency. "Assad's men are killing anyone within the military, police or others who don't obey their orders blindly."
Human rights activists and residents said troops began bombarding Jisr al-Shughour early on Sunday. Helicopter gunships were also seen hovering overhead.
An army general told the BBC the troops were only after those responsible for the killings of security personnel earlier this week. The army said the bodies of 10 security personnel were recovered from a mass grave.
Correspondents who witnessed the exhumation said four had been beheaded, and most of the bodies were riddled with bullet wounds.
The Local Co-ordination Committees, a group that documents the protests, said that over the past two days government snipers had killed at least 10 people in the village of Ariha.
There are continuing but unverifiable reports of army defections, with the latest saying an officer and 50 men had changed sides rather than fire on civilians in Jisr al-Shughour.
Meanwhile, the Syrian regime imposed travel bans on the former governor and head of security in the southern city of Deraa, an early epicentre of protests.
Atef Najib, who ran the security forces there, is a cousin of President Assad.
The White House on Monday condemned the latest crackdown in the "strongest possible terms".
The BBC's Owen Bennett-Jones, who is on the Turkish side of the border with Syria, says more than 5,000 refugees have registered with officials.
But thousands more may have entered the country unofficially, he says, while many are grouped at the border, awaiting the army's next move.
On the Turkish side, four camps have been set up for those fleeing.