Fighters from Al Qaeda's Syrian affiliate and allied militants advanced today on a key government-held military base in northwestern Idlib province, a monitor said.
The advance follows the capture of provincial capital Idlib by Al Nusra Front and its allies on Saturday.
"Violent clashes have been underway since Thursday night between the army and Al Nusra and its allies... around the Mastuma base," the Britain-based Syrian Observatory for Human Rights said.
The opposition groups had launched a "preventative attack on the base" where regime reinforcements were arriving ahead of a possible bid to recapture Idlib city, he said.
With the fall of Idlib city, regime forces in the province control only two towns and a handful of districts, along with the Abu Duhur military airport and five military bases.
IS launched an assault on Wednesday on the camp, after Palestinian groups inside arrested several jihadists.
They have reportedly captured a square where the camp's 18,000 residents usually gather to receive aid handouts.
"UNRWA remains acutely anxious about the humanitarian impact of continuing armed conflict between armed groups inside Yarmuk," spokesman Chris Gunness said.
The Observatory said government forces were shelling parts of Yarmuk intensively while Palestinian fighters battling IS militants were running low on ammunition.
Yarmuk has been under government siege for more than a year.
But Syrian state media said the camp was under "the control of terrorist groups", adding that there had been no army presence in Yarmuk "for a long time".
It gave no further details. Syria's government and state media refers to all those seeking to oust President Bashar al-Assad as "terrorists."
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