Syria’s government asserted Monday that 120 soldiers had been killed by armed protesters in the northern town of Jisr al-Shughour, amid indications that what began as a peaceful protest movement is turning into an armed rebellion in at least some parts of the country.

While there had been earlier, isolated reports of armed clashes between protesters and security forces elsewhere in Syria, this is the first time the government has accused the protest movement of killing so many of its soldiers in a single incident.

Activists and human rights groups said the number was almost certainly an exaggeration and was likely to be used as a pretext for an even harsher crackdown on the protest movement. Accounts from witnesses suggested that some form of violent confrontation had occurred, although there was no independent confirmation of the details or death toll.

The state-controlled Syrian Arab News Agency said the government was dispatching reinforcements to the area after the killings. It blamed the attacks on “armed gangs,” a phrase the government often uses to describe the almost overwhelmingly peaceful protest movement that has emerged in recent weeks to present the Syrian regime with the biggest challenge to its survival in three decades.

Jisr al-Shughour, which is near Syria’s border with Turkey, has become the latest focus of a harsh government campaign in response to the 11-week-old uprising against President Bashar al-Assad and his regime. Some Syrian activists said they believed the Syrian government was using the episode to justify even harsher measures against the town at a time when the United States is reported to be stepping up an effort to seek tougher sanctions against Syria at the United Nations.

“The regime is determined to use more violence, and it is scared of growing international pressure,” said Wissam Tarif of the human rights group Insan. “So the regime strategy is to portray this as a violent rebellion.”

Some Syrian activists who were interviewed by telephone confirmed that some residents of the town had taken up arms to defend themselves. A human rights activist outside the country said that he had spoken to people who told him that a swath of villages around Jisr al-Shughour is now outside government control.

“It is something not like the protests we have seen before,” said the activist, who insisted that he not be named because of the sensitivity of the subject. “Something very sad is happening, and it is against the spirit of the peaceful protests.’’

The Syrian news agency said that soldiers had been killed in several attacks, including an ambush, a bombing attack and a storming of security posts. Some Syrian activists disputed the government’s version of events, and although those details given by officials could not be verified, it appeared that at least some protesters did fight back.

A witness in Jisr al-Shughour said that several Syrian soldiers had been killed by protesters who took up arms to defend themselves against an onslaught of attacks by tanks and warplanes. He said the killings had taken place Sunday, when Syrian troops staged a major offensive against the town, which was repelled by armed protesters.

“We do not deny shooting back,” said the witness, a participant in the protest movement who spoke by telephone on the condition of anonymity because he fears for his safety. “But you must know that we only shot back after they started killing us. They killed 35 in a few hours.”

“We killed some,” he added. “The others were shot in the back for trying to defect. We took their weapons from them and turned the muzzles towards them. We shot and killed some. But not 80. They killed many more than we did.”

Jisr al-Shughour lies on a sectarian fault line between the Sunni-dominated agricultural plains to the east and a concentration of villages in the mountains to the west that are populated by Alawites, the minority sect to which Assad and his regime belong. The government has been distributing weapons to Alawite villagers to fight the mostly Sunni protest movement, with the goal, human rights activists say, of provoking a sectarian conflict that would justify the use of full force against protesters.

There have already been signs that Syrian protesters in the town of Tal Kalakh, on Syria’s borders with Lebanon, have acquired arms, and there have been reports of protesters firing back on scattered occasions in other areas of northwestern Syria where sectarian tensions are running high between Sunnis and Alawites.

It would not be surprising, said Andrew Tabler, a Syria expert with the Washington Institute of Near East Policy, if more protesters in areas subjected to particularly brutal methods took up weapons to defend themselves.

“The regime is being stretched thin and it’s losing its grip, and this is probably going to be the future of the conflict,” he said. “Once things take off like this, it’s hard to extinguish.”

Some Syrian activists have expressed concern that the more violent turn might reflect a stronger hand by the Islamist movement in what has until now been a largely secular upheaval demanding democratic reforms. But it remains unclear what role, if any, Islamists might be playing in the unrest; the Muslim Brotherhood was almost completely eliminated from Syria during an earlier, armed uprising in the early 1980s, which was brutally suppressed by the current president’s father, Hafez al-Assad.

There have been other indications in recent days that the Syrian authorities may be losing their grip in certain key protest flash points. After shooting dead more than 60 demonstrators in the town of Hama on Friday, the security forces retreated, for reasons that were unclear.

A video posted online Sunday showed protesters apparently hanging a captured member of the Syrian security forces in a central square, with no attempt by the authorities to intervene.

Another video posted on YouTube on Saturday showed uniformed Syrian security forces planting weapons on the bodies of dead civilians in the southern town of Daraa, the first town to openly rebel against the government and the first to be suppressed by tanks. The video illustrated, activists say, the lengths to which the government will go to portray the uprising as an armed revolt.

A special correspondent in Beirut contributed to this report.