A group of defectors calling themselves the Free Syrian Army is attempting the first effort to organize an armed challenge to President Bashar al-Assad’s rule, signaling what some hope and others fear may be a new phase in what has been an overwhelmingly peaceful Syrian protest movement.

For now, the shadowy entity seems mostly to consist of some big ambitions, a Facebook page and a relatively small number of defected soldiers and officers who have taken refuge on the borderlands of Turkey and Lebanon or among civilians in Syria’s cities.

Many of its claims appear exaggerated or fanciful, such as its boasts to have shot down a helicopter near Damascus this month and to have mustered a force of 10,000 to take on the Syrian military.

But it is clear that defections from the Syrian military have been accelerating in recent weeks, as have levels of violence in those areas where the defections have occurred.

“It is the beginning of armed rebellion,” said Gen. Riad Asaad, the dissident army’s leader, who defected from the air force in July and took refuge in Turkey.

“You cannot remove this regime except by force and bloodshed,” he said, speaking by telephone from the Syria-Turkey border. “But our losses will not be worse than we have right now, with the killings, the torture and the dumping of bodies.”

His goals are to carve out a slice of territory in northern Syria, secure international protection in the form of a no-fly zone, procure weapons from friendly countries and then launch a full-scale attack to topple the Assad government, echoing the trajectory of the Libyan revolution.

In the meantime, the defected soldiers are focusing their attention on defending civilians in neighborhoods where protests occur, while seeking to promote further defections, he said.

If the group achieves even a fraction of those aims, it would mark a dramatic turning point in the six-month standoff between a government that has resorted to maximum force to suppress dissent and a protest movement that has remained largely peaceful.

There is still scant evidence that the defectors are anywhere close to presenting a serious threat to Assad. Diplomats and activists say it is clear that the Free Syrian Army does have a presence in several locations, including the central city of Homs, the remote northern area of Jabal Zawiya near the Turkish border, and the eastern town of Deir al-Zour.

There have been frequent reports of firefights between defected soldiers and the regular army in these areas, but the numbers involved do not appear to be as large as the Free Syrian Army claims.

“I don’t think the numbers are big enough to have an impact one way or another on the government or on the contest between the protesters and the government,” said U.S. Ambassador Robert Ford, speaking by telephone from Damascus. “The vast majority of protests are still unarmed, and the vast majority of protesters are unarmed.”

There are nonetheless signs that the Free Syrian Army is expanding and organizing as reports of violent encounters increase. The group has announced the formation of 12 battalions around the country that regularly post claims on the group’s Facebook page, including bombings against military buses and ambushes at checkpoints.

One of the most active units is the Khalid Bin Walid Brigade in Homs, where the presence of hundreds and perhaps as many as 2,000 defected soldiers is believed to be responsible for an intensified government offensive over the past two weeks in which neighborhoods have been shelled and dozens of civilians have died.

According to defected soldiers and local activists, soldiers there are abandoning their units on a near-daily basis, encouraged in part by a tactic that involves ambushing patrols, shooting their commanders then convincing the rank and file to switch sides.

The brigade also serves as a defense force in neighborhoods opposed to the government, guarding streets while protests take place and attacking the militias, known as shabiha, that are an integral part of the government’s efforts to suppress dissent.

“We only kill them in self-defense,” said a captain in the brigade, interviewed via Skype, who requested that his name not be used, to protect his family from retribution.

He and other defected soldiers say they have Kalashnikovs, rocket-propelled grenades and antiaircraft guns and can count on a steady supply of ammunition secured from sympathetic soldiers within the military. News reports of arms seizures on both the Lebanese and Iraqi borders suggest weapons are also being smuggled from neighboring countries.

Though several activists and defected soldiers offered similar accounts of the Free Syrian Army’s activities, verifying them is impossible, because the Syrian government refuses to allow foreign journalists access to the country.

The Free Syrian Army has an interest in amplifying its activities to encourage defections. Activists committed to preserving the revolt’s pacifism have a stake in playing down its relevance.

The only admission by the government that defections are taking place has come in the form of a televised “confession” by one of the most prominent defectors, Lt. Col. Hussein Harmoush, who disappeared under mysterious circumstances in Turkey in late August then surfaced two weeks later on Syrian state television denouncing the opposition.

Defections are not new, but until now most have consisted of small groups of disgruntled soldiers fleeing orders to shoot civilians, then taking refuge in local homes, where they are hunted down and captured or killed, often along with those who sheltered them.

The phenomenon was causing so many civilian casualties that protest organizers this summer appealed to soldiers to not defect until they could count on sufficient numbers to make a difference, said Wissam Tarif, an activist with the human rights group Avaaz.

Soldiers with the Free Syrian Army say they are hoping that point has now been reached. Large-scale or high-ranking defections are still unlikely, because the overwhelming majority of the officer corps belongs to Assad’s minority Alawite sect, said a defected first lieutenant who has taken refuge in the Lebanese border town of Wadi Khaled and makes frequent clandestine visits to Homs to support the Free Syrian Army’s activities.

But among ordinary Sunni conscripts, frustration is building after six months of battling protesters. Many thousands of soldiers are deserting their units and going home simply because they want to see their families, said the officer, who uses the pseudonym Ahmad al-Araby to protect his family.

Asaad, the dissident general, predicted that the sectarian imbalance within the army will ultimately tilt the battle in the defectors’ favor.

“Ninety percent of the soldiers are Sunni, and their morale is bad,” he said. “Every day they are defecting, and the regime is in a panic because they know they are being destroyed from within.”