Iraqi troops storm IS-held Ramadi

After a months-long effort to cut off supply lines to Ramadi, Iraq's armed forces have stormed the Sunni Muslim city on the river Euphrates.

Iraqi soldiers plant the national flag over a government building in Ramadi as security forces advance their position in northern Ramadi, 70 miles (115 kilometers) west of Baghdad, Iraq. Source: AAP

Iraq's armed forces have stormed the centre of Ramadi, a spokesman for the counter-terrorism units says, in a drive to dislodge Islamic State militants from their remaining stronghold in a city they captured in May.

The operation to recapture Ramadi, a Sunni Muslim city on the river Euphrates some 100km west of Baghdad, began in early November after a months-long effort to cut off supply lines to the city, whose fall to IS was a major defeat for Iraq's weak central government.

Progress has been slow because the government wants to rely entirely on its own troops and not use Shi'ite militias in order to avoid rights abuses such as occurred after the recapture of the city of Tikrit from the militants in April.

US officials have also cautioned against the use of Iran-backed Shi'ite militias in retaking Ramadi from the hardline Sunni militants to avoid fanning sectarian tensions.

The Baghdad government has said it also wanted to spare civilians and give them the opportunity to leave the city.

"Our forces are advancing toward the government complex in the centre of Ramadi," the counter-terrorism units' spokesman Sabah al-Numani said on Tuesday.

"The fighting is in the neighbourhoods around the complex, with support from the air force."

Iraqi intelligence estimates the number of IS fighters entrenched in the centre of Ramadi, capital of Western Anbar province, at between 250 and 300.

The offensive to capture the city centre started at dawn, said Numani. Military units crossed the Euphrates river into the central districts using a bridge that was destroyed by the militants and repaired by army engineers, he said.

"Crossing the river was the main difficulty," he said. "We're facing sniper fire and suicide bombers who are trying to slow our advance, we're dealing with them with air force support."

If the attack to capture Ramadi succeeds, it will be the second major city after Tikrit to be retaken from IS in Iraq.

IS also controls Mosul, Iraq's second largest city, and Falluja, which lies between Ramadi and Baghdad.

Retaking Ramadi would provide a major psychological boost to Iraqi security forces after IS seized a third of Iraq, a major OPEC oil producer and US ally, last year.