Iraq's armed forces stormed the centre of Ramadi on Tuesday, a spokesman for the counter-terrorism units said, 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 100 km (60 miles) west of Baghdad, began in early November after a months-long effort to cut off supply lines to the city, whose fall to Islamic State 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.

"Our forces are advancing toward the government complex in the centre of Ramadi," the counter-terrorism units' spokesman Sabah al-Numani said. "The fighting is in the neighbourhoods around the complex, with support from the air force."

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

"It's a ferocious fight, it's premature to say how long it will take but we can say victory will be achieved in a few days," Numani said.

Dozens of militants had been killed, said Brigadier Gen. Yahya Rasool, spokesman of the joint operations command, declining to give a casualty toll for the armed forces.

Dawn offensive

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 fixed by army engineers, and another floating bridge set up to bring in more forces, 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 Islamic State in Iraq.

Islamic State 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 Islamic State seized a third of Iraq, a major OPEC oil producer and US ally, last year.

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