BAGHDAD (AP) — Iran's Revolutionary Guard says a U.S. drone strike killed two of its advisers near the Iraqi city of Tikrit, though the U.S. said Monday its coalition conducted no airstrikes against the Islamic State group in the area during that time.

U.S. Central Command said it didn't target the area around Tikrit from March 22 through March 24, the window when the Guard said the two men were killed.

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 as the men supported Iraqi forces trying to retake the Islamic State-held city of Tikrit. It identified the dead advisers as Ali Yazdani and Hadi Jafari, saying they were buried Sunday. It did not elaborate 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 surveillance around Tikrit, Saddam Hussein's hometown, on March 21 in support of large-scale operations to retake it after Iraqi efforts had stalled. The coalition said it began airstrikes on Tikrit the night of March 25.

The U.S. Air Forces Central Command 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.

Another U.S. official said Monday that America had seen no information to back up Iran's claims, but officials were looking into it. The official was not authorized to discuss the issue publicly and spoke on condition of anonymity.

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, the 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.

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Karimi reported from Tehran, Iran. Associated Press writers Adam Schreck in Dubai, United Arab Emirates, Qassim Abdul-Zahra in Tikrit, Iraq, and Lolita C. Baldor in Abington, Pennsylvania, contributed to this report.