BEIRUT (Reuters) - Working under the cover of darkness, their faces wrapped in scarves and black masks, defectors from Syria's army are still a bit of an unknown quantity in the rise of an armed challenge to President Bashar al-Assad.
Thousands of soldiers who fled the regular army after it started cracking down on an eight-month protest movement have formed nebulous armed units loosely linked to the umbrella "Free Syrian Army," led by officers hiding in Turkey.
Isolated inside cities across Syria, the fighters work like a shadowy guerilla operation, promoting slogans like "victory or death" and "death before humiliation."
The deserters say they turned their guns against the state to protect peaceful protests in which some 3,500 are estimated by the United Nations to have been killed. The government, which says it has lost 1,100 security forces to the fighters, calls them foreign-backed "terrorist" groups.
But it is hard for anyone to know much about these faceless men -- most journalists are barred entry into the country, and even protesters, whose demonstrations are surrounded daily by these armed masked guards, are unlikely to know their identity.
"We have to hide what we do, other than with a few trusted activists. Covering our faces is necessary -- if our identities were revealed, our families could be tortured or killed," said Mazen, a deserter who spoke to Reuters by telephone.
Mazen refused to give his location or describe his role in his armed unit: "The secret police are always listening."
Fighters attack soldiers and checkpoints in protest hotbeds across Syria almost daily, but say it is only to prevent harm to protesters. They also launch attacks to seize weapons.
Damascus, under pressure by the Arab League to stem bloodshed or face sanctions, has continued to crack down on the revolt in what it calls an act of self-defense.
Scattered reports of attacks on the Syrian army or intelligence units are as old as the protest movement, but a more organized force of deserter attacks began in September.
Colonel Riad al-Asaad, who heads the Free Syrian Army, told Reuters over 15,000 men have joined him, though some opposition sources say only 2,000 of them are armed. Either way, their numbers are dwarfed by Syria's regular army which is roughly 200,000-strong.
It is unclear how much control the Free Syrian Army has beyond that of an umbrella group. The units are relatively independent and say frequent cuts of phone lines and the Internet make communication with each other tough.
Even more challenging is finding ways to coordinate without detection by intelligence forces.
"It's a daily challenge to find ways to get in touch. We also have to work at night, and that helps give us a little advantage against the army," Mazen said.
New units appear often, and many have tried to stay in contact with followers in recent months through Facebook.
"We swear to Assad and his oppressive supporters we will chase you down, anywhere and any time ... we will regain dignity and freedom for our people," shout the "Hawks of Damascus," hoisting rocket-propelled grenades and Kalashnikovs in a video on Facebook announcing their unit's formation on November 17.
Activists support the need for the armed deserters -- rejecting the idea that they have tarnished what began as a peaceful protest movement.
"The Free Syrian Army has become a necessity," said Omar Idlibi, a prominent Syrian activist working in Lebanon. "Their protection allows us to preserve our peaceful protests."
Regular Syrians in the opposition welcome the defectors but with a mix of enthusiasm and caution. Reports of attacks raise morale for some. Others fear increased potential for violence that could easily spiral into sectarian strife or chaos.
"I don't sense any one ideology uniting these groups -- their personal background is influencing how they act," said Hassan, a Syrian opposition sympathizer who recently fled to Beirut. "Some places have sectarian strife or just a conservative attitude, and their battalions keep those ideas."
Reports of killings between majority Sunni Muslims and minority Alawites, the same sect to which the Assad family belongs, are on the rise in the central city of Homs, a center of armed revolt which has seen daily attacks on security forces.
While many units have names such as the "Hawks" or "Freedom Brigade," many also name themselves the "Al-Farouq" and the "Khalid bin al-Walid" battalions, after Islamic figures.
"They don't help the revolution by using those Islamic names. Why Khalid bin al-Walid? What will the Christians think? Of course they'll stay by the regime's side," said Abdullah, a Syrian from Damascus.
Colonel Asaad himself says his troops need to be more diverse to reflect the ethnic and religious make-up of Syria itself.
"We hope our brothers from the Alawites, the Druze and the Christian religions and our Kurdish brothers will join," he told Al Jazeera television. "No one should be afraid."
Some deserters working for the Free Syrian Army denied their groups' Islamic names were provocative and blamed sectarianism on attacks by "shabbiha" -- pro-Assad, Alawite militia men.
"The reality is the majority of Syria is Sunni Muslim, but we haven't been acting out of sectarianism. That's a card the government plays by using shabbiha to anger people," said Ali, a defector working for the Free Syrian Army from exile in Jordan.
Ali fled his regular army unit four months ago.
"We are not sectarian ... but if the government wants to make it sectarian, I will accept it and keep fighting them."
Activists privately complain that the group's message is confounded by a plethora of disorganized Facebook and Twitter pages run by each battalion. Some groups announce attacks which are denied by the Free Syrian Army hours later. Others post gruesome videos that appear to show beaten and bruised army officers confessing to killing protesters.
"There is a lot of chaos right now," said one activist working with the Free Syrian Army, who declined to be named. "It will take them some time to organize."
The Internet is vital, activists say, both for networking and raising morale through videos and posts of attacks.
The Facebook page attributed to the Free Syrian Army even uses its site to ask for contributions, routinely posting the details of a Turkish bank account.
It's unclear where the money goes -- fighters say it is used to buy food and arms from dealers inside Syria. But Damascus accuses its neighbors, particularly Turkey, of allowing weapons smuggling across the border to its opponents.
Relying on Facebook doesn't diminish the risks for fighters -- or their followers. The Internet's anonymity means intelligence forces can make look-alike pages to track unwitting followers or steal their information with spyware.
"I'm sure they're making fake pages," said Hassan, browsing battalion pages. "It's scary, we never know what we can trust."
Editing by Sonya Hepinstall