WASHINGTON—The U.S. has started providing Iraq with aerial intelligence in the stalled battle to oust Islamic State from Tikrit, drawing the American military into closer coordination with Iranian-backed militias spearheading the offensive.

Military officials said they aren’t working directly with Iran. But the intelligence will be used to help some 20,000 Iranian-backed Shiite militia fighters who make up the bulk of the force that has been struggling for weeks to retake the strategic city.

The Pentagon said Tuesday that the campaign had stalled amid tough fighting by hundreds of Islamic State fighters still holding most of Tikrit. At the request of the Iraqi government, the U.S. military began providing aerial video of the city, said an official with the American-led coalition helping Iraq fight Islamic State.

The U.S. involvement could pave the way for American airstrikes in Tikrit, hometown of onetime dictator Saddam Hussein.

The real-time aerial video surveillance sets the stage for the U.S. military to play a deeper role in the fight for Tikrit—if Iraq reduces its reliance on Iranian support.