WASHINGTON - The United States and Iran, which dueled viciously over Iraq during the years of US occupation, suddenly seem to be working in tandem as they confront what both see as a common, even mortal enemy: Islamic State.
Air strikes by Iran inside Iraq in recent days are only the latest manifestation of an increasingly muscular role by Tehran in Baghdad's war against Sunni militants. During the administration of George W. Bush, such actions would be denounced as meddling.
"I think it's self-evident that if Iran is taking on ISIL in some particular place and it's confined to taking on ISIL and it has an impact, it's going to be - the net effect is positive," Secretary of State John Kerry said on Wednesday, using an alternate acronym for Islamic State.
The change in tone is noteworthy in a relationship that has been acrimonious, and at times lethal, since Iran's 1979 Islamic revolution.
It comes against the backdrop of the most intensive US-Iran contacts in years, as Washington and other world powers try to convince Tehran to limit its suspected nuclear weapons programme.
It also comes as Washington and Tehran brace for a long conflict in Iraq. Some US officials go beyond accepting Iran's air strikes, crediting Tehran for what they say was its critical role in helping stop the advance of Islamic State in June - by mobilizing Iraqi Shi'ite militias - after the group seized the city of Mosul and appeared to threaten Baghdad.
That's a far cry from during the height of the US occupation of Iraq, when US officials charged that Shi'ite militias backed by Iran were killing and maiming American troops.
Much remains murky about the Iranian air strikes. Kerry did not confirm them publicly, and Iran denied striking Islamic State targets in Iraq, saying Tehran only provides military advice to the Iraqi government in its fight against the group.
But several US officials confirmed the air strikes, which took place in recent days and appear limited to a strip of territory in eastern Iraq, and said they were the first of their kind.
A US national security source, speaking on condition of anonymity, said the strikes, by 1970s-era US-made Phantom F-4 aircraft flown from Iranian bases, began about two weeks ago.
The US military stopped short of casting any judgment on the Iranian air raids, instead merely warning against inflaming sectarian tensions in Iraq.
"We're not going to take a position on that," said Colonel Steve Warren, a Pentagon spokesman.
US officials said privately that Iran's dispatch of jets over eastern Diyala province was not setting off alarms within the US government. Shi'ite Iran has steadily exerted its influence in Iraq, home to a Shi'ite-led government.
That includes military activity.
"We are looking at this with clear eyes," said one US official, speaking on condition of anonymity, acknowledging that it was unrealistic to think that neighbouring Iran would take a hands-off approach to the Iraqi unrest.
A second official said bluntly: "It's not surprising."
"It's no secret that Iran has had military assets, resources and activities, inside Iraq," the official said.
The United States was well aware of previous Iranian spy flights over Iraqi territory, one official noted. Iran-backed militias in Iraq are also among some of the most fearsome fighters there.
Reuters has previously reported on the presence in Iraq of military advisers from the Quds Force, the branch of Iran's Islamic Revolutionary Guard Corps that handles operations outside Iran and oversees Tehran's Iraqi militias.
Concurrent US and Iranian military activities in Iraq raise a host of questions as the United States moves to double the number of US troops on the ground and is poised to step up its own air strikes once Iraqi forces are capable of mounting major offensives.
But officials point out that places where Iran has been active have not been those where the United States is using its air power or has advisers on the ground.
"We're not in coordination with the Iranian military, before, during or after their military activities," the second official said.