Iraqi militia forces that have led the fight against Islamic State militants in Tikrit balked at U.S. intervention Thursday, saying that they would stop thousands of fighters under their influence from joining an offensive on the city.

Though the militia leaders said they would remain in their positions around Tikrit, their refusal to continue fighting raises the question as to whether regular Iraqi troops can continue the battle on their own.

The largely Iranian-backed paramilitary groups reacted with fury Thursday after coalition planes launched 17 airstrikes on Tikrit during a first wave of attacks. One Shiite militia, Kitaeb Hezbollah, which is designated by the United States as a terrorist organization, said it would go as far as shooting down any plane belonging to the U.S.-led coalition in the area.

Washington has pushed for Shiite militias to leave the battlefield, even as it is drawn into a fight against their enemy, the ­Islamic State militants. But the Shiite militias, many of which are hostile to the United States, play a dominant role among the Iraqi forces. Around Tikrit, they outnumber the regular Iraqi government troops by more than 6 to 1.

As the United States joins the conflict, the ire of the militias could have wide repercussions in a country where lines between them and Iraqi security forces have become increasingly blurred, and Iranian-backed groups have grown in power. The discord has the potential to undermine Prime Minister Haider al-Abadi’s authority while drastically cutting the number of pro-government forces at Tikrit from as many as 30,000 to around 4,000.

Thousands of ground troops involved in the Tikrit offensive belong to what are known as “popular mobilization units” — loose affiliations of Shiite fighters and militiamen that formed after a call to arms from Iraq’s leading Shiite cleric. They are largely under the control of militia leaders despite government claims otherwise.

Even in the face of U.S. pressure to reduce militia influence, Iraqi officials had indicated that they planned to use at least part of the popular mobilizations in a new offensive for Tikrit, with Abadi mentioning them as he announced the second push on Wednesday night. But the units said they would refuse to participate, as militia leaders attempted to reassert their influence in the wake of the U.S. strikes.

“All the popular mobilization will refuse to fight until the American airstrikes stop,” said Moeen al-Kadhimi, head of the popular mobilization committee on Baghdad’s provincial council. “Let them try to do it without us. America is just trying to steal our victory.”

The fight for the city, around 100 miles north of Baghdad, is considered a crucial test in the wider fight to drive the Islamic State from strongholds in northern Iraq, including Mosul, which has been the center of Islamic State power in Iraq since the militants took it last summer.

The offensive for Tikrit has stalled for more than a week, after initially advancing with relative ease across surrounding Salahuddin province, leaving fighters stalled around the city.

“If the public mobilization units aren’t going to fight, it’s an embarrassing break of solidarity,” said Michael Knights, an analyst at the Washington Institute for Near East Policy. “This is exposing a rift that already existed.”

While it will be a tougher fight without the popular mobilizations, what the Iraqi forces lose in manpower they make up for in intelligence and strike capacity, he said.

With only a small patch of ground to cover, the Iraqi army — joined by federal police and counterterrorism forces — has the potential to complete the operation, with coalition air support, Knights added. U.S. officials estimate that there are only around 300 Islamic State fighters in the town.

Gen. Lloyd J. Austin III, commander of U.S. Central Command, told U.S. senators Thursday that about 4,000 members of Iraqi government security forces were taking part in operations in Tikrit west of the Tigris River. He said Shiite militias and popular mobilization forces were not taking part in those operations; he said the militias had “pulled back.”

Austin said the U.S. government had imposed preconditions for launching airstrikes, including information on which forces were on the ground and a “credible scheme of maneuver” for the operation.

In contrast, Iraq’s defense minister, Khaled al-Obeidi, said there had been no change in the operation’s ground forces at the behest of the United States.

He said that the popular mobilization units were under government control and that the ground offensive for Tikrit was due to begin “very soon.” He spoke at a news conference at an air base in Baghdad on Thursday morning as he saw off Iraqi air force pilots as they left for morning raids on Tikrit, joining coalition planes in their campaign.

Militia representatives said the decision not to fight was their own. Khadami said leaders of the mobilization units and Shiite militias had held an emergency meeting Thursday before coming to the decision to remain in their positions but not participate in operations.

“We will be ready to launch an offensive when the coalition forces stop bombing,” said Naim ­Abboudi, a spokesman for Asaib Ahl al-Haq, one of Iraq’s most prominent Shiite militias. “We are suspending activities.”

Citing the widely held belief in Iraq that the United States has been dropping supplies to Islamic State militants, Kitaeb Hezbollah said it would consider any plane from the U.S.-led coalition a target.

“We have the capabilities to shoot them down,” said Jafar ­al-Husseini, a spokesman for the group.

Shiite cleric Moqtada al-Sadr’s Peace Brigades, which had only just arrived in Salahuddin after their leader previously froze their operations and didn’t fight in the battle, was the only militia to announce it was withdrawing its men.

Karen DeYoung and Missy Ryan in Washington contributed to this report.

Read more:

Today's coverage from Post correspondents around the world