BEIRUT -- Rebel-on-rebel fighting between an al-Qaida-linked group and an array of more moderate and ultraconservative Islamists has killed nearly 500 people over the past week in northern Syria, an activist group said Friday, in the most serious bout of violence among opponents of President Bashar Assad since the civil war began.
The clashes, which pit fighters from a variety of Islamist and mainstream factions against the al-Qaida linked Islamic State of Iraq and the Levant, have spread across four provinces in opposition-held parts of the north. The infighting has overshadowed the battle against the government, which in recent months has clawed back some of the ground it has lost to the rebels.
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The Britain-based Syrian Observatory for Human Rights said Friday that at least 482 people have been killed in fighting between the factions since Jan. 3. It said 157 were from the Islamic State of Iraq and the Levant, 240 from more moderate factions and 85 were civilians.
The Islamic State of Iraq and the Levant and another al-Qaida linked group known as Jabhat al-Nusra, or the Nusra Front, initially joined forces with moderate rebels fighting to oust Assad in a conflict that began in March 2011 as a popular uprising but morphed into a civil war.
The extremists proved well-organized and efficient fighters, giving the ragtag rebels a boost. But the "Islamic State" has alienated many Syrians, including other Islamist factions, over the past several months by using brutal tactics to impose its strict interpretation of Islamic law. The Nusra Front has largely stayed on the sidelines of the ongoing clashes, and in some places has tried to broker a truce.
The collection of ultraconservative rebels and more moderate brigades have made headway against fighters from the "Islamic State" in several areas of the provinces of Aleppo, Idlib, Hama and Raqqa -- although the al-Qaida-linked group has managed to regroup and curb some of its losses.
The Observatory reported renewed clashes Friday in Aleppo, Idlib and Raqqa.
Fighting between rebels and Assad's forces also continued.
Army troops killed dozens of rebels who tried to break the military's siege on opposition-held areas of the central city of Homs, state media and anti-government activists said Friday.
For months, Assad's forces have blockaded opposition-held areas of Homs as the military tries to squeeze the last pockets of resistance in the city known as the "capital of the revolution."
The SANA state news agency said the military killed at least 37 rebels in the Matahen area of Homs. It did not specify when they were killed or provide figures on government casualties.
The Observatory put the death toll at 45, and said the rebels were killed late Wednesday and early Thursday. Observatory director Rami Abdurrahman said the toll could rise because the number of anti-Assad fighters involved in the clashes numbered as many as 65.
Abdurrahman said he had no information on government deaths.
In Damascus, meanwhile, a U.N. official warned that the humanitarian situation in Palestinian-dominated Yarmouk district was deteriorating as aid access to the area remains cut.
Rebels seized Yarmouk more than a year ago, part of a swath of neighborhoods around Damascus now held by opposition fighters. Before the war, Yarmouk was a densely populated district of cheaply built multistory homes, but was called a "camp" since Palestinians came there as refugees during the 1948 Mideast war surrounding Israel's creation.
Christopher Gunness, an official with the U.N.'s Relief and Works agency that supports Palestinian refugees, cited reports of widespread malnutrition and the absence of medical care, including for pregnant women.
"The profound civilian suffering in Yarmouk deepens," he said.
"Residents, including infants and children, have been subsisting for long periods on diets of such things as stale vegetables, animal feed and cooking spices dissolved in water," he said. "Infants are suffering from diseases linked to severe malnutrition, including anemia, rickets, and kwashiorkor."
Gunness said electricity and heating cut for long periods, and the water supply is inconsistent at best.
"The imperative remains that the Syrian authorities and other parties must allow and facilitate safe and open humanitarian access into Yarmouk to enable us to assist civilians trapped there," Gunness said.