TIKRIT, Iraq - The U.S.-led coalition battling the Islamic State group has begun surveillance flights over the northern Iraqi town of Tikrit, a senior coalition official said Tuesday, marking the first time the alliance has taken part in a major offensive there that is being spearheaded by Iranian-backed Shiite militias.

The official said the flights and intelligence sharing began Saturday and were requested by the Iraqi government. He declined to comment on whether the coalition was carrying out airstrikes, saying he could not discuss current or future operations. He spoke on condition of anonymity because he was not authorized to speak to journalists.

Until now, the offensive to take back Tikrit, Saddam Hussein's hometown, has largely has been waged by Iraqi troops and Shiite militias advised by Gen. Qassem Soleimani, commander of the elite Quds Force of Iran's Revolutionary Guard. The coalition official declined to discuss whether U.S. forces were directly communicating with Iranians on the ground there.

Both the United States and Iran view the Islamic State group as a major threat but insist they are not coordinating their actions. The U.S. had previously said it was not taking part in the Tikrit offensive because it had received no request to do so from Baghdad.

The U.S.-led air campaign, launched in August, has allowed Iraqi forces to halt the IS group's advance and claw back some of the territory it seized last summer.

But the growing Iranian presence on the ground has complicated the mission, with Washington refusing to work directly with a country it views as a regional menace. The prominent role of the Shiite militias in the fight to retake Tikrit and other parts of Iraq's Sunni heartland, meanwhile, has raised concerns that the offensive could deepen the country's sectarian divide and drive Sunnis into the arms of the Islamic State.

A senior Iraqi military official said the coalition was not providing airstrikes in support of the Tikrit operation, but launched airstrikes on the nearby oil-refinery town of Beiji on Tuesday.