U.S. troops have come "under fire" and have "returned fire" in and around Mosul as allied forces try to retake the city from ISIS, a U.S. military spokesperson said on Wednesday.

“When someone is shooting at you, that is combat,” said Col. John Dorrian, the Baghdad-based spokesman, to Pentagon reporters.

While Dorrian said U.S. troops are operating “very close” to the front lines, he also added they do their best to avoid it, although circumstances can change where exactly the front lines are.

A separate U.S. defense official told Fox News that U.S. troops fighting in Mosul have been wounded in recent fighting, but declined to say how many, or when the incidents occurred.

The official told Fox News some were airlifted by helicopter for treatment, but declined to describe the wounds in any detail.

The news comes days after the U.S. commander of American and allied forces in Iraq and Syria told Fox News' Jennifer Griffin and other traveling press in Baghdad that U.S. troops were operating closer than ever to the front lines.

ISIS is estimated to have between 1,000 and 3,000 fighters in Mosul, Dorrian said.