Iraqi security forces advanced Tuesday into the centre of Ramadi city, which has been under the control of Islamic State group militants since May, a security official said.

"We went into the centre of Ramadi from several fronts and we began purging residential areas," said Sabah al-Noman, spokesman of the Iraqi counter-terrorism service.

"The city will be cleared in the coming 72 hours," he said.

The fresh push was launched overnight and is meant to result in the full recapture of Ramadi, the capital of Iraq's western province of Anbar.

The fighting in Ramadi is led by the elite counter-terrorism force, backed by US-led coalition air strikes and also supported by forces from the police, the army and Sunni tribes opposed to the militants.

ISIS has lost several key towns in Iraq since Baghdad and the autonomous Kurdish region started fighting back following the militants group's devastating offensive 18 months ago.

The Shiite-dominated Hashed al-Shaabi paramilitary forces were heavily involved in the battles that led to the recapture of towns such as Tikrit and Baiji but they have remained on the fringes in the battle for Ramadi.

Retaking the city, an insurgent bastion that saw some of the deadliest fighting against US troops a decade ago, would be the Iraqi federal forces' most significant victory so far.