By Khaled Oweis
AMMAN (Reuters) - Fighting erupted Friday between Syrian military intelligence agents and residents in the eastern city of Deir al-Zor after the killing of five protesters, witnesses said, in what appeared to be a serious armed challenge to President Bashar al-Assad.
Popular unrest against four decades of repressive rule by the Assad family, now in its fifth month, is taking on sectarian overtones with protesters from the Sunni Muslim majority pitted against minority Alawites dominating the power elite.
Military intelligence, in charge of securing loyalty to Assad among the army's mostly Sunni rank and file, has been spearheading a crackdown in Syria's Sunni tribal east, a strategic oil-producing region near the border with Iraq.
"Fighting is concentrating in the northwest of Deir al-Zor. It has been going on nonstop since 2 a.m. (2300 GMT)," a resident, who declined to be named, told Reuters by telephone.
"Tanks entered the city overnight, but there is talk of entire army units defecting. Electricity and communications have been cut," he said with the crackling of heavy machinegun fire audible in the background.
Residents earlier reported tank shelling in Deir al-Zor.
There have been individual instances of Syrians using weapons during the unrest, for example defending their homes during assaults on restive cities by security forces.
But the fighting reported in Deir al-Zor appeared to represent an armed response by a significant number of people to Assad's iron-fisted clampdown on public dissent.
Syrian authorities have expelled most independent journalists since the uprising began, making it difficult to verify reports of clashes, and do not usually comment on reports of killings.
Sunday, Assad replaced the civilian governor of Deir al-Zor province with a secret police officer, two days after the biggest pro-democracy demonstrations in the province so far.
Last week the army surrounded the town of Albu Kamal on the easternmost edge of Deir al-Zor after 30 soldiers defected following the killing of four protesters, residents said.
Deir al-Zor produces most of Syria's daily oil output of 380,000 barrels but is among the poorest of the country's 13 provinces, afflicted by drought and state mismanagement.
The official Syrian news agency said saboteurs "targeted" an oil pipeline near the central city of Homs Friday, without giving details of the nature of the attack.
Homs hosts one of Syria's two oil refineries and has been hit by big street protests. Assad has deployed tanks in Homs.
The Syrian leadership blames "armed terrorist groups" for most killings during the revolt, which began with demands for political freedoms and now seeks the toppling of Assad, who succeeded his late father, Hafez al-Assad, in 2000.
The global activist group Avaaz said in a new report that Syrian security forces have killed 1,634 people while at least 2,918 people had disappeared in Assad's violent crackdown. Another 26,000 people have been arrested, many of whom were beaten and tortured, and 12,617 remain in detention, it said.
The Syrian government has said more than 500 soldiers and security personnel have been killed. Human rights campaigners say soldiers who have refused to fire on civilians have been shot dead. They add that army conscripts and rank and file members have been defecting in increasing numbers.
Assad has relied on ultra-loyalist security units, which are mostly Alawite and commanded by his dreaded brother Maher, to quell the uprising.
Overnight Friday, witnesses said they saw around 2,000 republican guards being transported around Damascus ahead of Friday prayers in the biggest such deployment against possible protests in the capital since the uprising started.
In Madaya near the capital, residents told Reuters two civilians were killed in a security sweep. Madaya has witnessed large anti-Assad disturbances despite the stationing of armored vehicles in the area.
(Reporting by Khaled Yacoub Oweis, Amman newsroom; editing by Mark Heinrich)