A fourth Shi'ite militia group said it would remain in the battle but vowed to attack foreign members of the US-led coalition, raising the possibility that it might turn antiaircraft fire against US planes from what had been Iraqi positions.

The militia groups, some of which had Iranian advisers with them until recently, withdrew in protest of the US military airstrikes, which began late Wednesday, insisting that the Americans were not needed to defeat the extremists in Tikrit.

AL RASHID AIR BASE, Iraq — Three major Shi'ite militia groups pulled out of the fight in Tikrit against the Islamic State on Thursday, immediately depriving the Iraqi government of thousands of their fighters on the ground even as US warplanes readied for an expected second day of airstrikes there.


US military leaders were seen as likely to welcome the exit of the Shi'ite groups, so long as enough Iraqi fighters remain to keep the pressure on the Islamic State's holdouts. Before starting the airstrikes, US officials demanded that Iranian officials and the militias closest to them to stand aside, and had expressed concerns about sectarian abuses in areas controlled by the Shi'ite militias.

But too great or abrupt a withdrawal by militia forces, analysts said, could complicate the entire Iraqi counteroffensive. Even with the militias involved, officials said the progovernment force would not be large enough to help retake Mosul from the Islamic State.

Together, the Shi'ite groups that were pulling out represent more than a third of the 30,000 fighters on the government side in the offensive, analysts said.

"We don't trust the American-led coalition in combating ISIS," said Naeem al-Uboudi, spokesman for Asaib Ahl al-Haq, one of the three groups saying they would withdraw from the fight near Tikrit. "In the past they have targeted our security forces and dropped aid to ISIS by mistake," he said.


One of the commanders of the biggest Shi'ite militia, the Badr Organization, also criticized the American role and said his group might pull out.

"We don't need the American-led coalition to participate in Tikrit. Tikrit is an easy battle; we can win it ourselves" said Mueen al-Kadhumi.

"We have not yet decided if we will pull out or not," he said. The Badr Organization's leader, Hadi al-Ameri, was shown on Iraqi Television leading the ground fight Thursday in Tikrit.

The office of Prime Minister Haider al-Abadi announced Thursday night that he went personally to Tikrit, presumably to persuade Ameri to keep his fighters in the field.

The Badr Organization fields the largest cohesive ground force in the conflict, and its withdrawal from Tikrit would be potentially catastrophic, according to Wafiq al-Hashimi, head of the Iraqi Group for Strategic Studies.

"Abadi rushed into this decision to liberate Tikrit with the Americans without taking time to work out a compromise among all these groups . . . most of whom have a lot of disputes with the Americans," Hashimi said.

The US airstrikes in Tikrit began late Wednesday night and continued for 8½ hours, subsiding at dawn on Thursday, when Iraq's handful of Russian-made fighter jets took over for a succession of daytime raids.