WASHINGTON -- The list of countries bombing Islamic State targets in Iraq has thus far featured a host of classic United States partners -- Canada, the U.K., France. Now, it looks like the U.S. has a new quasi-partner in the air: Iran.

The U.S. is aware of Iranian bombing activity in the same national airspace where planes aligned with the U.S.-led coalition against the Islamic State are operating, a defense official told The Huffington Post Monday evening.

The official said he believes the Iranian bombing is unlikely to end as long as the Shiite-dominated nation feels threatened by the Sunni extremist group, also called ISIS. The bombing will not require a U.S. response unless Iran presents an immediate threat to U.S. forces in the air, he said.

"We are aware of that. I wouldn't say we're necessarily concerned with it -- we kind of have our eyes on it," the official said. He noted that the Iranian bombing has been taking place near the Iranian border, in a different part of Iraq than most U.S. and coalition activity. The official said he could only confirm reports of the bombing on the condition of anonymity.

While previousreports have said that Iran has provided weapons and equipment to the Iraqi government, the official's comments represent the first confirmation that Iran's own air force is involved in the fight in Iraq against the Islamic State.

The fact that the U.S. is not challenging this level of Iranian involvement is the strongest evidence yet that the Obama administration sees the Iranian government as a tactical partner in the Middle East. The stance is controversial, given that U.S. allies -- including Israel and Arab states helping tackle the Islamic State, like Saudi Arabia and the United Arab Emirates -- feel threatened by Iran.

The defense official's comments offered evidence for recent claims about an Iranian jet in Iraqi skies made by IHS Jane's Defence Weekly, a British defense analysis firm, and Haaretz, a leading Israeli newspaper. The plane, both outlets said, was spotted in Al Jazeera video footage from late November. They said it was assisting the Iraqi military -- a key U.S. on-the-ground ally -- in its biggest campaign against the Islamic State since the summer.

Neither outlet could verify whether the jet was Iranian: Both noted that only Iran and Turkey, a member of the U.S.-led coalition that has yet to militarily target the Islamic State, fly the kind of jet seen in the video. The outlets also differed slightly on their timelines for the sighting, with IHS Jane's dating the footage to Nov. 30 and Haaretz saying on Dec. 1 that it was from "a few days ago."

The official said the U.S. became aware of Iranian bombing in Iraq "earlier than" the recent claims, and that the U.S. military is aware that the planes are part of Iran's air force.

The sheer extent of U.S. involvement in Iraq makes undetected Iranian air activity there unlikely, he said. "We are there pretty heavily in the air if you include [intelligence collection], so just looking at those facts we have a pretty good picture of what's happening in Iraqi airspace. We would not be operating in Iraq without having a clear picture."

The White House declined to comment on the defense official's statement or the previous reports. The State Department did not immediately respond to a request for comment.

Iran began providing Iraq with assistance to face the threat of the Islamic State months before the U.S. started targeting the group in August.

Despite the U.S. involvement, Tehran still wields broad influence over Iraq's government and with some of Iraq's most effective fighting forces, brutal Shiite militias that are not formally aligned with the government but are committed to destroying the Islamic State. In September, a top Iranian commander threatened to "attack deep into Iraqi territory" if the Islamic State threatened Iran's borders.

U.S. officials told HuffPost last month that in both nations where the U.S. is targeting the Islamic State -- Iraq and Syria -- it knows it must play by Tehran's rules. This is particularly important, one official said, because of a fear that Iranian forces or their Shiite militia proxies might turn on the increasing number of Americans now on the ground in Iraq. An added concern is the future of ongoing diplomacy over Iran's nuclear program, which could, if it yields fruit, serve as the major foreign policy achievement of President Barack Obama's second term.

White House Press Secretary Josh Earnest said in November that the U.S. and Iran were not cooperating in the fight against the Islamic State. But he added that both countries, longtime opponents that have not had diplomatic relations since 1979, were interested in defeating the group -- and that representatives from the two nations had discussed their respective efforts against it.

"We won't share intelligence with them, but their interest in this outcome is something that's been widely commented upon and something that on a couple of occasions has been discussed on the sidelines of other conversations," Earnest said.

The defense official who confirmed the Iranian involvement said that because of Iran's relationship with the Iraqi government, pushing back against Iran's presence in Iraqi airspace is not an option for the U.S.

"We are there at the invitation of the Iraqi government," he said, referencing Iraq's request over the summer for international help against the Islamic State. "It's not like it was in 2010 [during the U.S. occupation of Iraq]: We're there at the invitation of the Iraqi government, so it's not for us to say what they should allow, what they shouldn't allow."

"It's their country. Our only concern would be if there are any type of force protection issues. Like I said, we have a pretty good understanding of what's in the air."

Analysts and opponents of Iran's role in the Middle East have warned that though Iran and the U.S. presently share a mutual threat, they likely do not have similar long-term visions for the region.

Joseph Bahout, a visiting scholar at the Carnegie Endowment for International Peace and a former adviser to the French foreign ministry, told HuffPost last month that he believes Iran has likely decided what it wants the Middle East to be: "a quagmire of open low-intensity fighting for years."