The northern Syrian town of Jisr al-Shughour is bracing for an assault by the military after the government said 120 security forces personnel had been killed there by "armed gangs".
The government says it will act "with force" to restore control.
Activists say the source of the violence is unclear, possibly involving a military mutiny.
Residents have posted messages on Facebook saying they fear a slaughter and appealing for help from outside.
Activists insist the uprising against the regime of President Bashar al-Assad is peaceful and scorn the government's talk of armed gangs.
'Shot in the back'
Communications were cut to the area around the town on Monday and the details of the attack were impossible to verify. Syria does not allow foreign journalists to report on events.
But Syrian state television said hundreds of gunmen had taken over Jisr al-Shughour, which lies about 20km (12 miles) from the Turkish border.
It said "armed gangs" ambushed police and 20 officers died. It said another 82 personnel were killed when the town's security headquarters were overwhelmed, eight in a bomb attack on a post office and 120 overall.
Opposition activists said there had been fighting but the situation was unclear and they feared the government was simply setting the stage for a new onslaught.
One told the Associated Press news agency that dozens of people had been killed in the past 24 hours - "a mix of civilians, police and security forces".
The BBC's Jim Muir in Beirut says some activist websites have suggested dissent within the security forces, even a mutiny.
He says YouTube carried footage, said to be from the town, showing several dead bodies described as belonging to soldiers who had been killed by security forces for refusing to fire on the people.
One resident told BBC Arabic: "We don't have any kind of weapons. The soldiers were coming our way, then they were shot in the back by some Syrian security elements."
State television says many civilians from the town have fled to nearby Latakia, and it carried interviews with people calling on the army to clear Jisr al-Shughour of "armed gangs" they said had been terrorising the place.
However, in messages posted on Facebook, residents appealed for help from the outside world, saying the "regime's gangs" were preparing to move in.
Our correspondent says one video posted on YouTube appeared to show the town square with a huge message written on the road saying "Jisr al-Shughour doesn't want the army to come in".
The government admitted it had lost control for "intermittent periods of time".
But Interior Minister Ibrahim Shaar vowed: "We will deal strongly and decisively, and according to the law, and we will not be silent about any armed attack that targets the security of the state and its citizens."
An Islamist uprising in Jisr al-Shughour in 1980 against the late President Hafez al-Assad was brutally crushed with scores of deaths.
Meanwhile, France says it is ready to ask the UN Security Council to vote on a draft resolution condemning Damascus, despite a likely Russian veto.
Speaking in Washington, Foreign Minister Alain Juppe said Paris believed that strong support in the 15-member Council might persuade the Russians "to change their mind".
"We think that it will be possible to get 11 votes in favour of the resolution and we'll see what the Russians will do," he said.
The draft resolution was drawn up by France, Britain, Germany and Portugal. The document condemns violence at the hands of the regime of President Assad and asks him to open Syrian cities to humanitarian teams.
