AL RASHID AIR BASE, Iraq — Three major Shiite militia groups pulled out of the fight against the Islamic State in Tikrit on Thursday, immediately depriving the Iraqi government of thousands of their fighters on the ground even as American warplanes readied for an expected second day of airstrikes there.
The militia groups, some of which had Iranian advisers with them until recently, pulled out of the Tikrit fight in protest of the American military airstrikes, which began late Wednesday night, insisting that the Americans were not needed to defeat the extremists in Tikrit.
A fourth Shiite militia group said it would remain in the battle in Tikrit, but vowed to attack foreign members of the American-led coalition, raising the possibility that it might turn anti-aircraft fire against American planes from what had been Iraqi fighting positions.
American military leaders were likely to welcome the withdrawal of the Shiite groups, so long as enough Iraqi fighters remain to keep the pressure on the Islamic State’s holdouts. Before starting the airstrikes, American 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 Shiite 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 current pro-government force would not be large enough to help take Mosul back from the Islamic State, also known as ISIS or ISIL.
Together, the four Shiite groups that were pulling out represent more than a third of the 30,000 fighters on the government side in the offensive against the Islamic State, analysts said.
“We don’t trust the American-led coalition in combating ISIS,” said Naeem al-Uboudi, the spokesman for Asaib Ahl al-Haq, one of the three groups which said they would withdraw from the front line around Tikrit. “In the past they have targeted our security forces and dropped aid to ISIS by mistake,” he said.
One of the leaders of the biggest militias in the fight, the Badr Organization, also criticized the American role and said his group, too, 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, who is one of the Shiite militia group’s top commanders.
“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 in Tikrit on Thursday.
The office of prime minister Haider al-Abadi announced Thursday night that he went personally to Tikrit, persumably to persuade Mr. Ameri to keep his fighters in the field.
The largest of the Shiite militia groupings, 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.
“Dr. Abadi rushed into this decision to liberate Tikrit with the Americans without taking time to work out a compromise among all these groups and the Americans, most of whom have a lot of disputes with the Americans,” Mr. Hashimi said.
Meanwhile, another Iranian-aligned Shiite militia group reacted with defiance and threats against the Americans. “We are staying in Tikrit, we are not leaving and we are going to target the American led coalition in Tikrit and their creation, ISIS,” said Akram al Kabi, the leader of the Al Nujabaa Brigade, a powerful militia that has previously sent fighters to Syria on behalf of the Bashir al-Assad regime there.
The American airstrikes in Tikrit began late Wednesday night and continued for eight and a half hours, subsiding at dawn on Thursday, when Iraq’s handful of Russian-made fighter jets took over from this base on the outskirts of Baghdad and further bombed Tikrit in a succession of daytime raids.
The other groups that announced they would boycott the Tikrit operation were Qatab Hizbullah, which like Ashaib Ahl al-Haq is closely aligned and supported by Iran, and the Peace Brigade, the latest name for a militia made of up followers of the Shiite cleric Moktada al-Sadr, previously known as the Mahdi Army.
Hakim al-Zamily, one of the leaders of the Sadr group, said his group had warned it would pull out of the Tikrit fight if the Americans were brought in. “We don’t trust the Americans, they have targeted our forces many times in so-called mistakes,” he said.
Mr. Sadr, whose troops fought bitter battles against the Americans during much of the Iraq war, said his group was pulling out because, in his words, “the participation of the so-called international alliance is to protect ISIS, on the one hand, and to confiscate the achievements of the Iraqis, on the other hand.”
Since March 2, Islamic State forces in Tikrit have been under attack by the Iraqi militias, collectively known as the Popular Mobilization Committees, and regular Iraqi military forces, together numbering more than 30,000 fighters.
Still, a much smaller force of Islamic State fighters has been able to hold them off in a few areas of the city for almost four weeks. In recent days, despite the claims of self-sufficiency made by militia commanders, Iraqi military officials said American airstrikes were needed to break the deadlock.
The militias who were withdrawing did not say they were quitting their positions in the Tikrit area altogether, or in adjoining areas of Salahuddin Province, just returning to their nearby bases and boycotting the front-line advance.
By 10 a.m. Thursday, the Iraqi jets had carried out four waves of attacks on Tikrit, consisting of up to five jets each from this base, taking over from the American bombers in the coalition.
As the Tikrit operation continued through Thursday, Staff Gen. Anwer Hamid, the commander of the Iraqi Air Force, said that for operational reasons, American aircraft would concentrate on night bombing runs, and the Iraqis would continue their daytime sorties.
“Their role in this fighting is very important to us,” he said. “They have a high number of aircraft and they have good capabilities, they can really help us,” he said.
While the Americans and their coalition partners have hundreds of aircraft at their disposal, the Iraqi Air Force mainly has a dozen Russian-made Sukhoi Su-25 attack jets.
The American-led military coalition has been carrying out airstrikes against ISIS since last summer in Kurdistan and around Mosul in northern Iraq, as well as in Syria, but had not joined the Tikrit offensive previously. The coalition has advisers and trainers in Iraq, but unlike the Iranians, it has not so far as is known sent them to front-line positions with Iraqi ground forces, at least in part because those forces have been dominated by irregular militiamen in the wake of last summer’s near collapse of the regular Iraqi Army.
A news release issued by the American military command late Wednesday said that “Iraqi security forces have ISIL in Tikrit encircled. Renewed efforts on the ground supported by the coalition are aimed at dislodging ISIL fighting elements from Tikrit and once again placing the town under the government of Iraq control.”
The American statement made no mention of the Shiite militia forces but stressed that the coalition would “continue supporting the Iraqi security forces and the government of Iraq.”
The deputy governor of Salahuddin Province, Ammar Hikmat, said that some of the remaining ISIS fighters were seen trying to swim across the Tigris River, but that some 150 fighters remained surrounded in a former palace of Saddam Hussein and in the center of Tikrit.
An Iraqi military official in Salahuddin, speaking on condition of anonymity as a matter of official policy, said that 73 ISIS fighters had been killed in the American bombing raids on the palace, while another 7 were killed trying to flee across the Tigris River.