The Iraqi city of Ramadi has been captured by Islamic State after days of intense fighting and bombings by the terror group, officials say.

"Ramadi has fallen," said Muhannad Haimour, spokesman for the governor of Anbar province.

"The city was completely taken ... It was a gradual deterioration. The military is fleeing."

He also confirmed the provincial command centre in the city had fallen into enemy hands.

A colonel among troops who had withdrawn added: "Daesh has just taken full control of all main security bases", using an Arabic acronym for IS.

But Iraqi Prime Minister Haider al-Abadi ordered government forces to "hold their positions".

He called on troops, tribesmen and other elite forces to push the militants out of Ramadi.

US Secretary of State John Kerry, speaking during an official visit to South Korea, said he was confident the loss of Ramadi would be reversed and re-iterated his warning that the fight against IS would be a long one.

An estimated 500 people - militants and civilians - have been killed in Ramadi over the last two days and around 8,000 have fled as IS launched a fierce offensive of suicide car bomb attacks.

Fifteen Iraqi troops and police died on Sunday in four near-simultaneous car bombings, with soldiers retreating and leaving behind vehicles and weapons.

A purported message by Islamic State on a jihadist internet forum also appeared to confirm the city was firmly in its grip.

"God has enabled the soldiers of the caliphate to cleanse all of Ramadi," read the message.

One officer inside the captured command base described being surrounded, with mortars "raining down" and IS fighters "in almost every street".

Local television also reported that Iraq's prime minister, Haider al Abadi, is sending in Shia militias and has urged troops to hold their positions until they arrive.

The militias have played a key role in fighting IS in other areas of Iraq but there are worries they could stir up sectarian violence in Sunni-dominated Anbar.

Islamic State's fresh offensive seized most of Ramadi on Friday, with their black flag raised over a local government headquarters.

A group of Iraqi special forces managed to hold out in the Malaab neighbourhood but security sources said they pulled out on Sunday after suffering heavy casualties.

Ramadi, a city of around half a million and just 110km (70 miles) from Baghdad, is one of a few places in the sprawling desert terrain of Anbar province that had managed to hold out against IS.

Fighting has been raging there since last year, but IS renewed their push for the city in April.

The terror group has taken control of large areas of Iraq and Syria, slaughtering civilians and destroying ancient artefacts, as it tries to extend its self-proclaimed Islamic religious state.

The apparent capture of Ramadi is a major setback in the Iraqi government's effort to drive back the group from areas it captured last year.

A US-led coalition has been pounding IS fighters from the air for months.

Seven more air raids attacked IS positions near Ramadi in the 24 hours up until Sunday morning, officials confirmed.

The Pentagon also said Delta Force commandos had killed a senior IS leader in a secret mission in Syria over the weekend.

In Syria meanwhile, worries of another destruction of ancient artefacts by the terror group appear to have eased.

The government said it had pushed back IS fighters from the ancient city of Palmyra, a UNESCO world heritage site.

Islamic State took over the north of the city on Saturday but provincial governor Talal Barazi said the "attack was foiled" and that "the situation in the city and its outskirts is good".

But UK-based monitoring group The Syrian Observatory for Human Rights said there were still reports of fighting in some areas.

Its director, Rami Abdel Rahman, said IS fighters remained just one kilometre from the ancient site.

Islamic State has already destroyed priceless treasures, which it says promote idolatry and are against Islam, at sites such as Nimrud in Iraq.
(Sky News)