BEIRUT — Armed men killed 120 Syrian security forces and torched government buildings Monday in a northern region where troops have unleashed deadly assaults on protesters for days, Syria said. The government vowed to respond “decisively,” hinting at an even more brutal crackdown by a regime known for ruthlessly crushing dissent.

If confirmed, the attack in the north would be a turning point in what so far has largely been a peaceful uprising threatening the 40-year rule of President Bashar Assad and his family.

Opposition activists were skeptical of the official casualty toll, saying the authorities were setting the stage for a new onslaught. But even they acknowledged there was fighting, although it was not clear who was involved. The government described the attackers as “armed gangs,” a phrase it often uses to describe the protesters.

Communications were cut to the area around Jisr al-Shughour on Monday and the details of the attack were impossible to verify, but there have been unconfirmed reports by activists and residents in the past of Syrians and even mutinous troops fighting back against security forces.

Adnan Mahmoud, the chief government spokesman, acknowledged that Syrian forces had lost control of some areas for “intermittent periods of time” and said residents had appealed to the army to do what was necessary to restore security.

“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,” said Interior Minister Ibrahim Shaar.

The surprisingly high death toll among pro-regime troops would suggest a major retaliation against the crackdown on the uprising – though by whom was unknown because of the communications seal over the area. Regardless of who carried out Monday’s attack, it shows new cracks in a rule that has held out through weekly protests of thousands of people.

Human rights activist Mustafa Osso said there were unconfirmed reports of a few soldiers who switched sides and were defending themselves against attacking security forces, but he said the reports suggest the mutiny is limited and “does not pose a threat to the unity of the army yet.”

“The protesters have so far been peaceful and unarmed,” Osso said.

A Syrian activist speaking on condition of anonymity said there were unconfirmed reports of infighting between security forces. “The situation is very foggy, it is unclear who is doing the shooting, but the situation is very serious and appears to be getting out of control,” he said.

He said there were fears the army was preparing to launch a major attack.

Before Monday’s killings, the government and some human rights groups said more than 160 soldiers and security forces had died in the uprising. The latest deaths marked by far the deadliest single strike against them.

The government’s unusual admission of the death toll and loss of control appeared to set the stage for an even stronger action to crush the popular uprising that began in mid-March and poses a potent threat to the Assad regime.

State television added the armed groups carried out a “real massacre,” mutilating some bodies and throwing others in the Orontes River.