Iran's Revolutionary Guard says a U.S. drone strike has killed two of its advisers in Iraq, though the U.S. said Monday it has only struck militants in its campaign against the Islamic State group.

The claim came as negotiators on Monday attempted to reach a deal on Iran's contested nuclear program, which hard-liners in the Islamic Republic have opposed as giving away too much to the West.

The Guard said on its sepahnews.ir website the strike happened March 23, just after the U.S.-led coalition began airstrikes to support Iraqi forces trying to retake the Islamic State-held city of Tikrit. It identified the dead as Ali Yazdani and Hadi Jafari, saying they were buried Sunday. It called them advisers, without elaborating on whether Iran contacted Iraqi or U.S. forces after the strike.

Iran occasionally reports on the death of its forces in Iraq and Syria, where it is backing embattled President Bashar Assad, but this is the first time Iran has said it has lost forces in an attack by the U.S. in those campaigns.

The U.S.-led coalition began a campaign around Tikrit, Saddam Hussein's hometown, on March 21 in support of large-scale operations to retake it after Iraqi efforts had stalled.

Reached by The Associated Press about the Guard's claim, the U.S. Embassy in Baghdad said: "The international coalition is aimed at Daesh only," using an alternate Arabic acronym for the Islamic State group.

"All airstrikes are carried out at the request of the Iraqi government and in full coordination with the (Iraqi) Ministry of Defense," the embassy said, without directly addressing the Iranian claim.

The U.S. Air Forces Central Command also recorded "no strikes by coalition aircraft, manned or unmanned, in the vicinity of Tikrit from 22 to 24 March," spokesman Col. Edward T. Sholtis said in response to a query by the AP.

The Islamic State group now controls a third of both Iraq and neighboring Syria. The U.S. began airstrikes against the group in August, while Iran has offered advisers and other assistance to Iraq to fight the extremists.

On Friday, Iraqi media outlets reported casualties among Iraqi security forces near the University of Tikrit, allegedly from U.S. airstrikes. But the U.S. Embassy in Baghdad denied those claims, saying "no coalition airstrikes took place during the time or in the vicinity of these alleged casualties."

The offensive to retake Tikrit largely has been waged by Iraqi troops and Shiite militias advised by Gen. Qassem Soleimani, commander of the Guard's elite Quds Force. Several Shiite militias announced Thursday that they would boycott the Tikrit operations due to U.S. involvement, but Army Gen. Lloyd Austin, head of U.S. Central Command, told a Senate hearing Thursday that the U.S. agreed to Iraqi government requests to support the operation on the condition that the militias wouldn't be involved.

The militias have been accused by various human rights groups of committing atrocities against Sunni civilians in areas recaptured from the militant group. On a visit Monday to Baghdad, United Nations Secretary-General Ban Ki-moon expressed his concern over "summary killings, abductions and destruction of property perpetrated by forces and militias fighting alongside Iraqi armed forces."

Iraqi military commanders said Monday they retook a hospital in southern Tikrit. The officials, who spoke on condition of anonymity because they were not authorized to brief journalists, said the military's three-pronged offensive is progressing toward the center of Tikrit slowly because of booby traps and suicide bomb attacks by the militants.