AL RASHID AIR BASE, Iraq — By Day 2 of the American airstrike campaign against militants holed up in Tikrit, the mission appeared beleaguered on several fronts on Thursday: Thousands of Shiite militiamen boycotted the fight, others threatened to attack any Americans they found, and Iraqi officials said nine of their fighters had been accidentally killed in an airstrike.

In Washington, American military leaders insisted that things were going according to plan. They said that they were stepping into the Tikrit fight only after the Iranian- and militia-led advance on the city had stalled after three weeks, and that they welcomed working solely with Iraqi government forces.

Gen. Lloyd Austin, the head of the United States Central Command, told a Senate hearing on Thursday that no Shiite militias remained in Tikrit.

While the withdrawal of Iranian-led Shiite militias was one of the preconditions for the Americans to join the fight against the Islamic State in Tikrit, the sudden departure of three of the major groups risked leaving the Iraqi ground forces short-handed, especially if other Shiite militiamen also abandoned the fight.