The Iraqi operation to take back Tikrit from Islamic State jihadists is the largest-scale offensive yet launched by Baghdad and Shiite militia backed by neighboring Iran were playing a key role.
But the US-led coalition, which has conducted thousands of air strikes against the IS group to back up Iraqi and Kurdish forces on the ground, is not providing air power for the latest Iraqi offensive, officials said.
"We're not providing air strikes in support of the operation around Tikrit," Pentagon spokesman Colonel Steven Warren told reporters.
The assault on IS in Tikrit included Iraqi aircraft but it was unclear if Iranian planes were also involved.
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Both Iraqi and Iranian media reported that Qassem Soleimani -- commander of the Al-Quds Force cover operations unit of Iran's elite Revolutionary Guards -- was in the area to help oversee operations.
Warren declined to say whether US surveillance aircraft were providing intelligence for the Tikrit operation.
But a Pentagon official, speaking on condition of anonymity, told AFP that American planes were assisting with intelligence and reconnaissance.
"We were aware of the operation before it started but I'm not going to get into the details of what degree of cooperation we conducted before this," he said.
"We're tracking it closely."
Asked about Iran's activities, he said: "We're aware that there is great Iranian interest in Iraq's fight against ISIL," using another acronym by which the group is known.
But he said he would not discuss the details of Iran's involvement in the conflict.