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 Shiite heartland.
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 holds a third each of Iraq and adjacent Syria.
Anbar provincial council member Azzal Obaid said hundreds of Shiite fighters, who had arrived at the Habbaniya air base last week after Islamic State took Ramadi, deployed into Khalidiya on Saturday and were approaching Siddiqiya and Madiq, towns in contested territory near Ramadi.
Disadvantaged by poor morale and cohesion among his security forces, Iraqi Prime Minister Haider al-Abadi, a Shiite, sent Shiite paramilitary groups to try to retake Ramadi despite the risk of inflaming tensions with Anbar's aggrieved, predominantly Sunni population.
Jaffar Husseini, spokesman for the Shiite paramilitary group Kataib Hezbollah, said it had deployed more than 2,000 reinforcements which had managed to secure Khalidiya and the road connecting 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 advancing towards 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. A pro-government Sunni tribal leader in the area said on Friday that Islamic State forces were around 5 km (3 miles) from Khalidiya.
Islamic State has controlled Fallujah for more than a year.
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.