Thousands of Shiite militiamen are being deployed to retake control of the strategically important city of Ramadi, the capital of Anbar province, from the Islamic State (Isis).

At least 3,000 Shiite-led fighters are being dispatched by the Iraqi administration after the Sunni insurgent group claimed control of most parts of the city, just 110kms northwest of Baghdad.

"The Iraqi army has formed two regiments of Anbar tribal fighters who had previously joined Hashd al-Shaabi [Popular Mobilisation Units] in Anbar and were being trained at the Habbaniyah military base east of Ramadi. The regiments consist of 1,000 fighters and all of them are from the Anbar tribes," a statement from Anbar's provincial council read.

Iraqi authorities had said the militiamen were stationed in the military base and were entering Ramadi from the south. The government says all the men are fully trained, well-equipped and capable of taking on IS militants.

Local reports quoting witnesses said IS insurgents were seen cruising towards the military base, south of Ramadi, in armoured vehicles after the Shiite men began to move.

The battle for Ramadi has been raging for the past few days after the Islamists launched a surprise attack on the city seizing government buildings.

The Iraqi security forces were pushed back and eventually fled the city due to the rapid advance of the militants. The capture of Ramadi is being widely interpreted as a major victory for IS in recent months and some analysts are equating it with the group's capture of Mosul last year.

Playing down the significance of the fall of Ramadi, the US said the security forces would regain the city.

"To read too much into this single fight (over Ramadi) is simply a mistake. What this means for our strategy, what this means for today, is simply that we, meaning the coalition and our Iraqi partners, now have to go back and retake Ramadi," said Pentagon spokesperson Colonel Steve Warren.

The US-led coalition said the air strikes near Ramadi have been stepped up with 19 targets being bombed near Ramadi in the past 72 hours.