WASHINGTON, March 11 (Reuters) - The top U.S. military officer on Wednesday predicted certain victory by Iraqi forces and Shi'ite militia battling to retake the city of Tikrit but voiced concern about how Sunni Muslims would be treated once Islamic State militants were driven away.
The remarks to Congress by General Martin Dempsey, chairman of the Joint Chiefs of Staff, came as Iraqi security forces and Iranian-backed militia advanced from the north and south to fight their way into Tikrit, former Iraqi leader Saddam Hussein's home city.
It was their biggest counter-offensive so far against Islamic State militants, and U.S. forces, despite their deep investment in Iraq's war, have been watching from the sidelines.
"There's no doubt that the combination of the Popular Mobilization forces and the Iraqi security forces, they're going to run ISIL out of Tikrit," Dempsey told a Senate hearing, using an acronym for the militant group.
"The question is what comes after, in terms of their willingness to let Sunni families move back into their neighborhoods, whether they work to restore the basic services that are going to be necessary, or whether it results in atrocities and retribution."
The Tikrit operation is the most visible demonstration yet of how the United States and Iran, which dueled viciously over Iraq during the years of U.S. occupation, seem to be working in tandem against Islamic State.
If Iraq's Shi'ite-led government retook Tikrit it would be the first city clawed back from the Sunni insurgents and would give it momentum in the next, pivotal stage of the campaign: recapturing Mosul, the largest city in the north.
Dempsey said the forces advancing on Tikrit were overwhelmingly composed of 20,000 Iranian-backed Shi'ite Muslim militias known as Hashid Shaabi (Popular Mobilization) units.
"I would describe them as Iranian-trained and somewhat Iranian equipped," Dempsey said, adding he had no indications they intended to attack the nearly 3,000 American forces now stationed in Iraq.
In addition, there was an Iraqi brigade of about 3,000 troops as well as a couple of hundred forces from the Iraqi counter-terrorist service, Dempsey said.
Dempsey, fresh back from a trip to Iraq this week, said the outcome of Tikrit would speak volumes about whether Iran would use its influence in Iraq constructively.
"The Tikrit operation will be a strategic inflection point one way or the other in terms of easing our concerns or increasing them," he said. (Additional reporting by Ahmed Rasheed in Baghdad and Patricia Zengerle in Washington)