In a shift, the coalition as of March 21 began providing intelligence from surveillance flights for the Tikrit assault, a senior coalition military official said earlier in Baghdad.
US officials in Washington confirmed the account of "an eye in the sky" for the Iraqi troops and Iranian-backed Shiite militia.
President Barack Obama's administration has insisted it does not coordinate military operations directly with Iran.
But the surveillance flights and discussions on possible US air raids in Tikrit illustrate how Washington is moving towards greater indirect collaboration with Tehran, despite the intense distrust between the two arch-foes.
IS led a sweeping offensive last June that overran large areas north and west of Baghdad, including Tikrit, the capital of Salaheddin province and executed dictator Saddam Hussein's home town.
Iraqi security forces initially fared poorly against IS, with multiple divisions collapsing in the north, but have since made major gains against jihadists with the aid of tens of thousands of allied paramilitaries, the US-led coalition and Iran.
- Balancing act -