Shi'ite Muslim militiamen and Iraqi army forces launched a counter-offensive against Islamic State insurgents near Ramadi on Saturday, a militia spokesman said, aiming to reverse potentially devastating gains by the jihadi militants.

The fall of Ramadi, the Anbar provincial capital, to Islamic State on May 17 could be a shattering blow to Baghdad's weak central government. The Sunni Muslim jihadis now control most of Anbar and could threaten the western approaches to Baghdad, or even surge south into Iraq's Shi'ite heartland.

Anbar provincial council member Azzal Obaid said hundreds of Shi'ite fighters, who had assembled last week at the Habbaniya air base, moved into Khalidiya on Saturday and were nearing Siddiqiya and Madiq, towns in contested territory near Ramadi.

Two police officers later told Reuters the pro-government forces, which they said included locally allied Sunni tribesmen, had advanced past those towns to within one kilometer of Husaiba al-Sharqiya, an Islamic State-run town 7 kilometers (4 miles) east of the Ramadi city limits.

One officer said the Shi'ite-led forces exchanged fire with Islamic State but there was no immediate word on casualties.

Jaffar Husseini, spokesman for Shi'ite paramilitary group Kataib Hezbollah, said more than 2,000 reinforcements had joined the pro-government advance and they had managed to secure Khalidiya and the road linking it to Habbaniya.

"Today will witness the launch of some tactical operations that pave the way to the eventual liberation of Ramadi," he told Reuters by telephone.

At the same time, Islamic State units have been pushing toward Fallujah to try to absorb more territory between it and Ramadi that would bring them closer to Baghdad, the Iraqi capital, around 80 km (50 miles) to the east.

Islamic State has controlled Fallujah for more than a year.

Ramadi's loss is the most serious setback for Iraqi forces in almost a year and has cast doubt on the effectiveness of the US strategy of air strikes to help Baghdad roll back Islamic State, which now holds a third each of Iraq and adjacent Syria.

Iraqi Prime Minister Haider al-Abadi, a Shi'ite, sent Shi'ite paramilitary groups out to Anbar to try to retake Ramadi despite the risk of inflaming tensions with the province's aggrieved, predominantly Sunni population.

But he had little choice given the poor morale and cohesion within government security forces.

A UN spokesman said on Friday that some 55,000 people have fled Ramadi since it was stormed by Islamic State earlier this month, with most taking refuge in other parts of Anbar, a vast desert province that borders on Syria, Jordan and Saudi Arabia.

ISLAMIC STATE FLAG AT PALMYRA CITADEL

In Syria, Islamic State fighters raised their flag over an ancient citadel in the historic city of Palmyra, pictures posted online overnight by the group's supporters showed.

The militants seized Palmyra, known as Tadmur in Arabic and strategically significant with nearby natural gas fields and roads leading southwest to Damascus, on Wednesday after days of heavy fighting with the Syrian army.

"Tadmur citadel under the control of the caliphate," read a caption on one picture posted on social media sites. In another, a smiling fighter is shown carrying the group's black flag and standing on one of the citadel's walls.

It was not possible to verify the images' authenticity.

US-led coalition forces have conducted a further 22 air strikes on Islamic State positions in Iraq and Syria since Friday, including near Ramadi and Palmyra, the US military said.

Palmyra is home to a UNESCO World Heritage site, and Syria's antiquities chief has said the insurgents would destroy its 2,000-year-old ruins, including well-preserved Roman temples, colonnades and a theater, if they took control of them. While hundreds of statues have been taken to safe locations, there are fears for larger monuments that cannot be moved.

Islamic State destroyed ancient monuments and antiquities they see as idolatrous in areas of Iraq they captured last year.

Supporters have also posted videos they say show the group's fighters going room to room in government buildings in Palmyra searching for government troops and pulling down pictures of Syrian President Bashar al-Assad and his father.

Some activists have said more than 200 Syrian soldiers died in the battle for the city in the center of Syria.