Fighters from Al-Qaeda's Syrian affiliate and allied militants advanced Friday 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 base, seven kilometres (four miles) south of Idlib city, is the biggest regime base in the province, said Observatory director Rami Abdul Rahman.

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.

The Observatory added that clashes were underway between the Islamist forces and government troops in the area around the Shiite-majority village of Fuaa.

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.

Elsewhere in Syria, jihadists from the Islamic State group have pushed further into the Palestinian Yarmuk camp in Damascus and now control 70 percent it, according to the Observatory.

IS launched an assault Wednesday on the camp, after Palestinian groups inside arrested several jihadists.

Palestinian fighters and Syrian rebels initially pushed back the IS assault but the jihadist counter-attacked and have made fresh advances since Thursday inside the camp.

They have reportedly captured a square where the camp's 18,000 residents usually gather to receive aid handouts.

The UN agency for Palestinian refugees UNRWA expressed concern about the situation, saying it had been unable to deliver aid to the camp since Wednesday.

"UNRWA remains acutely anxious about the humanitarian impact of continuing armed conflict between armed groups inside Yarmuk," spokesman Chris Gunness said.

"With intense fighting continuing into a third day, the lives and safety of the 18,000 Palestinian and Syrian civilians inside Yarmouk are substantially threatened."

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."