Iraqi forces regain ground from Islamic State militants in western Iraq, advancing towards the city of Ramadi one week after it fell to the insurgents. (Reuters)

Shiite militiamen and Iraqi army forces launched a counteroffensive against Islamic State insurgents near Ramadi on Saturday, a militia spokesman said, aiming to reverse potentially devastating gains by the jihadist militants.

Anbar provincial council member Azzal Obaid said hundreds of Shiite fighters, who had assembled last week at the Habbaniyah air base, moved into Khaldiyah 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 Husaybah, an Islamic State-run town four miles east of the Ramadi city limits.

One officer said the Shiite-led forces exchanged fire with the insurgents, but there was no immediate word on casualties.

Jaffar Husseini, spokesman for the Shiite paramilitary group Kataib Hezbollah, said that more than 2,000 reinforcements had joined the pro-government advance and that they had managed to secure Khaldiyah and the road linking it to Habbaniyah.

Displaced civilians from Ramadi, Iraq, wait on Friday to receive United Nations aid in a camp in the town of Amiriyat al-Fallujah, west of Baghdad. Thousands of residents fled Ramadi after it was taken by Islamic State militants. (Hadi Mizban/Associated Press)

“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, some 50 miles to the east.

The 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 U.S. strategy of airstrikes to help Baghdad roll back the Islamic State, which now holds a third each of Iraq and adjacent Syria.

Meanwhile, in Syria, pictures posted online overnight by the group’s supporters allegedly showed Islamic State fighters raising their flag over an ancient citadel in the historic city of Palmyra.

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.

U.S.-led coalition forces have conducted a further 22 airstrikes on Islamic State positions in Iraq and Syria since Friday, including near Ramadi and Palmyra, the U.S. 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.

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