Iranian-backed Shiite militias have left the northern Iraq city of Tikrit, a key condition the U.S. demanded before it agreed to begin launching airstrikes Wednesday to support Iraqi forces and try to retake the city from the Islamic State group, the top U.S. general for the Middle East said Thursday.

Army Gen. Lloyd Austin appeared to stun Senate Armed Services Committee members with that development during a morning hearing, telling the panel that the ground offensive in Tikrit led by Shiite militias, with help from Iran, had stalled.

Austin said he witnessed brutality by Shiite militias during his extensive experience leading U.S. and coalition forces in Iraq.

"I will not -- and I hope we will never -- coordinate or cooperate with Shiite militias," he said, adding a blunt explanation for why the Iranian-led offensive failed: The wrong approach was taken, he said.

"These forces obviously were not being controlled by the government of Iraq," he said, and they were poorly led.

Austin, who heads U.S. Central Command, said that there now are about 4,000 Iraqi forces, commandos and police battling for the city, and with American help, he believes the campaign to retake the city will move forward. The battle for Tikrit is widely seen as a step toward the more difficult and potentially decisive battle to regain control of the larger city of Mosul.

Asked if the Iranian forces are nearby, Austin said, "I'm sure they're still in the area." He said he believes they may be on the east side of the Tigris river, while Tikrit is on the west side. And he added that, as of his last update, Maj. Gen. Qassem Soleimani, commander of the Guard's elite and secretive Quds Force, was not in Tikrit or in that area.

A spokesman for Iraq's Popular Mobilization Units, which mostly consist of Iranian-backed Shiite militias, offered a different explanation.

Mu'een al-Kadhimy said that a number of Shiite militias, including Asaib Ahl al-Haq, Ketaeb Hizbollah and Muqtada al-Sadr's Peace Brigades, are boycotting the Tikrit operation because of the involvement of U.S. forces.

"Their involvement is potentially harmful to the operation," he told The Associated Press, adding that the powerful Badr Brigade will most likely join the boycott. "We led the fight to liberate al-Dawr, to liberate al-Alam, and we are capable of liberating Tikrit without the help of American forces."

Mohammed Abu Ragheef al-Moussawi, the spokesman for Kataeb Hizbollah, told the AP that he knew "for more than 10 days that the Americans would try to participate and pressure the Iraqi government to abide by its rules. The American pressure was strong enough, and the government accepted whatever they demanded. But we told them from the start that if the Americans enter the fight for Tikrit, we will withdraw. That was our condition."

Iran had been providing artillery and other weaponry for the Tikrit battle, and senior Iranian advisers like Soleimani, have helped Iraq coordinate the offensive. U.S. officials estimate that two-thirds of the ground troops involved were Shiite militias; the others are combinations of regular Iraqi army soldiers and Sunni tribal fighters.

That growing Iranian presence complicated the mission, with Washington refusing to work directly with Tehran, even though it is currently embroiled with Iran in sensitive negotiations over a nuclear deal.

Austin told the committee that whether or not a nuclear deal is reached, he still considers Iran the greatest regional threat.

The prominent role of the Shiite militias in the fight to retake Tikrit and other parts of Iraq's Sunni heartland has also raised concerns that the offensive could deepen the country's sectarian divide and drive Sunnis into the arms of the Islamic State group.

Sen. John McCain, R-Ariz., began the hearing by asking Austin about the perceived 40,000 Shiite militia members in Tikrit, prompting the surprising retort from Austin announcing there were no Shiite militia in the city.

The Islamic State group seized the Sunni city -- Saddam Hussein's hometown -- last summer during its rapid advance across northern Iraq.

Austin said Iraqi leaders requested the airstrikes.

Tikrit is deemed an important test of the ability of Iraq, with coalition support, to retake ground it ceded to the Islamic State last year. The U.S. initially did not provide air support in Tikrit because Baghdad pointedly chose instead to partner with Iran in a battle it predicted would yield a quick victory.

The U.S.-led coalition conducted airstrikes in 17 locations Wednesday, hitting Islamic State group checkpoints, buildings, staging areas and a command and control facility using fighter jets, bombers and drones, according to U.S. Central Command.