Local fighters in a Palestinian refugee camp in Damascus advanced Wednesday in clashes with Islamic State militants two weeks after the extremist group stormed the area in its deepest foray into the Syrian capital, a Palestinian official and an activist said.

Damascus-based Palestinian official Khaled Abdul-Majid said that Palestinian factions have forced the IS fighters to retreat from some positions in Yarmouk camp.

Sami Hamzawi, a Palestinian activist from the camp who currently lives outside Syria, said the Hamas-affiliated Aknaf Beit al-Maqdis group has captured several buildings and is currently advancing from areas it holds northeast of Yarmouk. Hamzawi said he is in regular contact with camp residents.

Islamic State fighters overran much of Yarmouk earlier this month, establishing a foothold in the Syrian capital for the first time. Palestinian and Syrian officials have vowed to retake Yarmouk, a built-up area that housed 160,000 Palestinians and Syrians before the civil war.

"Because of pressures by the Palestinian factions on Daesh fighters, they were forced to retreat from some posts," Abdul-Majid said, using a derogatory Arabic acronym to refer to IS.

The Britain-based Syrian Observatory for Human Rights, which has a network of activists around Syria, reported intense clashes in the camp. It said Aknaf Beit al-Maqdis is backed by some Syrian Islamic fighters.

Hamzawi said Aknaf Beit al-Maqdis fighters captured several buildings overlooking the Salahuddin al-Ayoubi mosque, one of the main mosques in the camp. "It seems they have retaken the initiative," Hamzawi said, referring to the Palestinian fighters in the camp.

The IS incursion that began on April 1 was the latest in a series of trials for Yarmouk's residents, who have endured starvation and disease amid a devastating two-year government siege.

Also Wednesday, Syria's state-run news agency SANA said government forces backed by warplanes have captured the villages of Kfar Najad and Nahliya in the northwestern province of Idlib.

The Observatory said militants launched a counterattack in the area in an attempt to retake positions captured by government forces.