BAGHDAD - Thousands of Iranian-backed Shiite militias have begun mobilizing on the east of the key Iraqi city of Ramadi to prepare for a counter-attack against Islamic State militants who captured it on Sunday.
White House spokesman Eric Schultz termed the capture of Ramadi, the capital of Anbar province, a "setback," and said the US has vowed to help Iraq reclaim it with continued airstrikes and military advice.
"We believe that Iraqi forces have the capacity to ultimately take Ramadi with coalition support," Schultz said.
"We will retake Ramadi," said Col. Steve Warren, a Pentagon spokesman.
The fall of the largely Sunni Ramadi, 110km west of Baghdad, is being considered a defeat for Iraqi security forces and military, which largely retreated on Sunday.
The Iraqi government had sought help from the Iran-backed militias after the military was routed in the face of the Islamic State advance that saw around 500 people being killed.
Leaders of the Shiite militias on Monday said their fighters were going to launch attacks on Ramadi in a bid to restore government control over the city. Their objective is to prevent Islamic State, which now controls Ramadi, Fallujah and Mosul, from advancing east toward capital, Baghdad, or south towards Shiite holy sites.
Around 3,000 of the Shia militia, known as Hashid Shaabi or Popular Mobilisation, have massed near Ramadi.
Prime Minister Haider al-Abadi called on Sunday night for the militias to join the battle. Anbar's provincial council also voted in favor of requesting support from the Shia militias, which the lawmakers said was necessary in light of the collapse of pro-government forces.
In response, the Islamic State militants have advanced towards the military base where the government-allied fighters have amassed.
There are concerns that the clash could whip up sectarian violence in the Sunni-dominated province.
Meanwhile, US warplanes have stepped up airstrikes against IS at the request of the Iraqi security forces.
The UN estimates that nearly 25,000 people have fled Ramadi since IS won the battle for the city.
Youssef al Kilabi, a spokesperson for the Shia militias fighting alongside the Iraqi forces, said they will "eliminate the barbaric enemy", and "achieve this triumph and we will not accept anything less than that".
"We will support the legitimacy in Iraq represented in the government and parliament," he added.
A spokesmen for the militia groups said plans were underway for the "battle of Anbar".
On Monday, Islamic State fighters tried to push closer towards Baghdad.
Battles flared on the outskirts of the village of Husaybah, about eight miles east of Ramadi and close to the Euphrates River, Washington Post quoted Rafia al-Fahdawi, a tribal elder from the village, as saying.
About 10 village defenders have been killed in the attacks on Husaybah, he said.
However, the bigger aim of the militants appears to be to clear a path for an assault on the Habbaniyah air base, about 20 miles east of Ramadi on the road to Baghdad.
Gen. Martin Dempsey, chairman of the US Joint Chiefs of Staff, said in a statement that the Islamic State's advance into Ramadi was "a serious setback for its long-suffering inhabitants. It is also a setback for the ISF," or Iraqi Security Force.
"We will continue to support Iraq's Security Forces with US airstrikes, training, and equipment," he said.