Syrian activists say Islamic militants have taken over the majority of neighborhoods in a Palestinian refugee camp on the edge of Damascus as fighting continues for the fourth day.

They say the militants include the Islamic State group and the Nusra Front, the al-Qaida affiliate in Syria. The two groups are rivals in Syria but appear to be cooperating in the attack on the Yarmouk camp.

An activist in southern Damascus, Hatem al-Dimashqi, said Saturday that rebel groups have launched a counteroffensive aimed at ousting the militants from the camp.

Islamic State militants stormed the Yarmouk camp on Wednesday, marking the extremist group's deepest foray yet into Damascus.

Damascus-based Palestinian official Khaled Abdul-Majid said Saturday that the Islamic State group was holding almost half of the camp.