TIKRIT, Iraq (AP) — 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 cannot discuss current or future operations. He spoke on condition of anonymity because he was not authorized to speak to journalists.

Up to 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 U.S. 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 has meanwhile raised concerns that the offensive could deepen the country's sectarian divide and drive Sunnis into the arms of the Islamic State group.

A senior Iraqi military official said the coalition is not providing airstrikes in support of the Tikrit operation, but launched airstrikes on the nearby oil refinery town of Beiji on Tuesday. He added that Soleimani had just left Tikrit after providing front-line assistance and advising since the start of the operation.

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"He will come back if we need him to," said the Iraqi official, who spoke on condition of anonymity because he's not authorized to brief journalists.

Iran has provided military advisers and weapons to Iraqi troops and Shiite militias, and last year it carried out airstrikes near its border in Iraq's Diyala province.

The Sunni city of Tikrit lies about 130 kilometers (80 miles) north of Baghdad. It is one of the largest cities held by the Islamic State group and lies on the road connecting Baghdad to militant-held Mosul, the country's second-largest city.

U.S. military officials have that said a coordinated mission to retake Mosul likely will begin in April or May and involve up to 25,000 Iraqi troops. But the Americans have cautioned that if the Iraqis are not ready, the offensive could be delayed. The U.S. and other coalition members have been providing weapons and training to Iraqi and Kurdish forces in the north in an effort to prepare them for the battle ahead.

The Tikrit operation is the largest to date and requires careful air and ground coordination in order to minimize civilian casualties and damage to the city's infrastructure.

Iraqi security forces and allied militias surrounded Tikrit and entered some northern and southern neighborhoods earlier this month. But the offensive has since been hindered by roadside bombs and snipers, and Iraqi officials say they will not rush a final assault.

Pentagon spokesman Col. Steve Warren said Tuesday that the Iraqi ground operation against the Islamic State group has "stalled" and that the Tikrit operation "has not moved forward recently."

"They certainly have not given up," he added. "I think the difference is, after you've been static for about a week you're stalled."

Iraqi forces have made progress in driving the Islamic State group out of rural areas and small villages, but populated areas like Tikrit could prove a greater challenge.

"Urban combat is exceedingly difficult combat, and the Iraqis are seeing that," Warren said.

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Salama reported from Baghdad. Associated Press writer Robert Burns in Washington contributed to this report. Follow Salama on Twitter at www.twitter.com/vmsalama .