WASHINGTON - Iranian jets have carried out air strikes against Islamic State militants in Iraq in recent days, Pentagon officials and independent analysts say, underscoring the strange alliances generated by the war against the extremist group that has beheaded Americans and killed and terrorized Iraqi civilians.

Washington and Tehran are locked in tough negotiations over Iran's nuclear program. But the two adversaries have been fighting parallel campaigns on the same side in Iraq to defend the Shiite-dominated government - and the region's Kurds - from IS militants who seized a large section of the country.

It has long been known that Iranian troops and advisers have been fighting alongside Iraqi forces, but until this week there had been no confirmation of Iranian air activity.

The timing and nature of the strikes are not clear, but a senior U.S. official said they occurred in Diyala province, which extends from northeast Baghdad to the Iranian border. The official spoke only on condition of anonymity because he was not authorized to disclose that information.

The Qatari-based broadcaster Al-Jazeera filmed a jet flying over Iraq on Nov. 30 that was identified by Jane's Defence Weekly as an American-made F-4 Phantom.

The Phantom, a twin-engine fighter-bomber that was sold to Iran's U.S.-backed shah in the 1970s, was last produced in 1981.

Iran in the 1980s fought a brutal, ultimately stalemated war with Iraq when that country was led by Saddam Hussein and his Sunni-controlled Baath Party. But the U.S.-led invasion that toppled Hussein left an Iraqi government closely aligned with Iran. A majority of Iraqis are Shiite, as are most Iranians.

Speaking in Brussels on Wednesday, Secretary of State John Kerry said that Iranian attacks on IS militants would represent a positive development.

"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, its net effect is positive," Kerry told reporters. "But that's not something we're coordinating."