BAGHDAD- A column of Shi'ite militia fighters today arrived at a military base near Ramadi as Baghdad moved to retake the western Iraqi city that has fallen to Islamic State militants in the biggest defeat for the government since last summer.

Setting the stage for renewed fighting over the city, Islamic State militants advanced in armored vehicles from Ramadi towards the base where the Shi'ite paramilitaries were massing for a counter-offensive, witnesses and a military officer said.

At the same time, U.S.-led warplanes stepped up raids against the Islamists, conducting 19 strikes near Ramadi over the past 72 hours at the request of the Iraqi security forces, a coalition spokesman said.

The Shi'ite militia, known as Hashid Shaabi or Popular Mobilisation, was ordered to mobilize after the city, the capital of Anbar province, was overrun on Sunday.

The militiamen give the government far more capability to launch a counterattack, but their arrival could add to sectarian animosity in one of the most violent parts of Iraq.

"Hashid Shaabi forces reached the Habbaniya base and are now on standby," said the head of the Anbar provincial council, Sabah Karhout.

An eyewitness described a long line of armored vehicles and trucks mounted with machine guns and rockets, flying the yellow flags of Kataib Hezbollah, one of the militia factions, heading towards the base about 30 km (20 miles) from Ramadi.

Spokesmen for militia groups said reconnaissance and planning were underway for the upcoming "battle of Anbar", the vast Euphrates River valley province where the U.S. military fought the biggest battles of its 11-year occupation.

Ramadi is dominated by Sunni Muslims. Prime Minister Haider al-Abadi signed off on the deployment of Shi'ite militias to attempt to seize back the area, a move he had previously resisted for fear of provoking a sectarian backlash.

About 500 people have been killed in the fighting for Ramadi in recent days and up to 8,000 have fled, a spokesman for the provincial governor said.

The city's fall marked a major setback for the forces ranged against Islamic State : the U.S.-led coalition and the Iraqi security forces, which have been propped up by Iranian-backed Shi'ite militias

It was also a harsh return to reality for Washington, which at the weekend had mounted a special forces raid in Syria in which it said it killed an Islamic State leader in charge of the group's black market oil and gas sales, and captured his wife.