By Tom Perry
BEIRUT (Reuters) - The Syrian arm of al Qaeda attacked a Western-backed rebel group near Aleppo on Thursday, the rebel group and an organization monitoring the civil war said, threatening one of the few remaining pockets of the non-jihadist opposition.
The Nusra Front seized positions from the Hazzm movement west of Aleppo, the Syrian Observatory for Human Rights said. The Nusra Front said it was forced to act after Hazzm detained two of its fighters and captured its weapons and offices.
Hazzm is one of the last remnants of non-jihadist opposition to President Bashar al-Assad in northern Syria, much of which has been seized by the Nusra Front and Islamic State - an offshoot of al Qaeda that controls a third of Syria.
Hazzm has received what it describes as small amounts of military aid from foreign states opposed to Assad, including U.S.-made anti-tank missiles. But it has lost ground to better armed and financed jihadists: Nusra drove Hazzm from the nearby Idlib province in October.
While U.S.-led air strikes have focused on pushing back Islamic State in eastern and northern Syria, the Nusra Front has deepened its influence in the northwest.
"They want to put an end to the Free Syrian Army," said a Hazzm official by telephone. "The Free Syrian Army" refers to an array of mainly Western-backed armed opposition groups that have little or no central coordination.
"There have been a number of confrontations before, but this is the biggest," said the official, who said he was speaking from northern Syria. The Nusra Front was mobilizing reinforcements, meaning Hazzm would have to withdraw fighters from front lines with government forces.
The Nusra Front said in a statement it tried to avoid escalation with Hazzm but the group recently detained two Nusra fighters. "The Front will take the final road to free its soldiers," it said.
The jihadist domination of the insurgency has complicated diplomatic efforts to end the nearly four-year conflict that has killed around 200,000 people, and frustrated U.S. efforts to train and a equip fighters to contain Islamic State.
The sway of the remaining non-jihadist groups is mostly restricted to small pockets in around Aleppo and southwestern areas of Syria near Jordan.
The Syrian military has meanwhile waged its own campaign against Islamic State, making gains in the last two days near an air base in the eastern province of Deir al-Zor, said the Observatory's Rami Abdulrahman.
Kurdish forces drove the last Islamic State fighters from the border town of Kobani, also known as Ayn al-Arab, earlier this week. Abdulrahman said the Kurdish militia known as the YPG advanced into five nearby villages in the last three days.
(Editing by Dominic Evans)