RAMADI, Iraq, May 18 -- The Islamic State (IS) militant group on Sunday took full control of the city of Ramadi, the capital of Anbar province, Iraq, after the Iraqi security forces withdrew from their positions, a provincial security source said.

The source told Xinhua that the military units stationed in the Eighth Brigade headquarters north of Ramadi pulled out fully towards the 160 kilometer area west of Ramadi and that the militants (Daash) took control of the headquarters.

The IS militants carried out fierce clashes with the security forces and allied Sunni tribal militias in northern Ramadi later Sunday, and managed to take full control of the city after the troops withdrew from the headquarters of the army's 8th Brigade, said the source on condition of anonymity.

He added that the IS militants also detained hundreds of families who failed to make their escape.

Earlier Sunday, the source said that the IS militants seized al- Mal'ab district and Iskan district in central Ramadi, and then pushed forward to other areas.

The IS militants launched a series of attacks in Ramadi since Thursday night and managed to seize the districts of Jamiyah and Albu Alwan in the city. On Friday, the militants expanded in the city and took control of the government compound in central on Saturday,.

On Saturday, the militants continued their push in the city and carried out several attacks, including suicide car bombings, despite the arrival of three brigades to the battleground in Ramadi to support the security forces and allied militias.

The Iraqi air force has provided air support for the security forces fighting in Ramadi, as well as the U.S.-led coalition warplanes which carried out several airstrikes against IS positions in and out of Ramadi, the source said.

The IS group has seized most of Iraq's largest province of Anbar and tried to advance toward Baghdad, but several counter attacks by security forces and Shiite militias have pushed them back.

The security situation in Iraq has drastically deteriorated since last June, when bloody clashes broke out between Iraqi security forces and the IS militants.