Iraq's armed forces have stormed the centre of the city of Ramadi in a fresh bid to oust Islamic State (IS) militants who seized the key city in May.

Key points: Iraqi forces storm Ramadi in a bid to recapture city from IS

Iraqi forces storm Ramadi in a bid to recapture city from IS Officials hope to clear IS forces within 72 hours

Officials hope to clear IS forces within 72 hours Snipers and suicide bombers attacking Iraqi troops

Snipers and suicide bombers attacking Iraqi troops US-led coalition backs army with air strikes

The major offensive to recapture the city, whose fall to IS was a major defeat for Iraq's weak central Government, started at dawn and was backed by US-led coalition air strikes.

"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.

"Our forces reached the Bakr neighbourhood. We did not face strong resistance, only snipers and suicide bombers and this is a tactic we expected."

The US military spokesman in Baghdad, Colonel Steve Warren, said he had no doubt the jihadists would be defeated.

"I think the fall of Ramadi is inevitable — the end is coming," he said.

"But that said, it's going to be a tough fight. The Iraqi security forces still have much work to do and still have hard fighting ahead of them."

Footage on state TV channel Iraqiya showed soldiers driving down the deserted streets of the bombed-out city, entering homes carefully to spot possible booby traps and retrieving shells and rockets from abandoned IS positions.

The Iraqi Government has been preparing the offensive for months, cutting off supply lines and repeatedly telling residents in the centre to leave, in a bid to spare civilian lives.

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

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

"We built temporary bridges on the Euphrates [river] and our forces were able to cross the river to enter residential areas and gain access to the city centre," a brigadier general said, speaking on condition of anonymity.

The fighting in Ramadi is led by the elite counter-terrorism force and is also supported by forces from the police, the army and Sunni tribes opposed to the jihadists.

IS has lost several key towns in Iraq since Baghdad and the autonomous Kurdish region started fighting back following the jihadist 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.

No more than 300 IS fighters left in city: military officials

IS fighters have had plenty of time to dig in since they took full control of Ramadi on May 17 after blitzing government forces with wave after wave of car and truck bomb attacks.

The jihadists built tunnels to move without being exposed to the coalition's daily raids but their supply lines were gradually severed and military officials estimated last week there no more than 300 fighters left in the city.

The breakthrough came earlier this month when counter-terrorism forces broke down IS defences and retook the key south-western neighbourhood of Al-Tameem.

After taking a few days to beef up their new positions, Iraqi military leaders had said in recent days a final push was imminent and leaflets urging the population to flee were dropped over the weekend.

"The distance between our forces and the governmental compound, which is located in the central district of Hoz, is less than a kilometre," said the brigadier general, adding that clashes were ongoing.

The provincial headquarters is believed to be one of the main IS bases in the city and was at the heart of deadly fighting earlier this year.

According to another military official, speaking on condition of anonymity because he is not authorised to talk to the press, 15 families managed to escape from Hoz in the past 24 hours.

"They were able to flee the lockdown imposed by Daesh on civilians and they found shelter with the army on the southern side of the city," he said, using an Arabic acronym for IS.

The operation to recapture Ramadi began early last month ( AFP: Ahmad Al-Rubaye )

The senior officer said the civilians were mostly children, women and elderly men, who were screened and then taken to a safe area on the edge of Ramadi.

Iraq's Defence Minister, Khaled al-Obeidi, and other officials said in recent days they believed there were still a few civilians in Ramadi being used as human shields by IS.

The US-led coalition said its aircraft carried out six strikes on IS targets in the Ramadi area on Tuesday.

"That's been the pace we've maintained for the past month," Colonel Warren said.

"We certainly encourage the Iraqi security forces to continue their push into the city and we'll provide them with the air support they need."

A victory in Ramadi would leave Mosul, Tal Afar and Fallujah as the only major Iraqi cities still in IS hands and further undermine the group's claim that the caliphate it proclaimed last year is expanding.

Mr Obeidi has said successive operations had shrunk the area controlled by IS from nearly 40 per cent of Iraq last year to 17 per cent.

The operation to recapture Ramadi began early last month after a months-long effort to cut off supply lines to the city ( AFP )

Reuters/AFP