Tehran says a U.S. drone strike killed two Iranian advisers in Iraq last week, but the United States says it has only struck Islamic State (IS) militants in its campaign.

Iran's Revolutionary Guard said on its sepahnews.ir website on March 30 that the strike occurred on March 23, just after the U.S.-led coalition began air strikes to support Iraqi forces trying to retake the IS-held city of Tikrit.

The website identified the dead as Ali Yazdani and Hadi Jafari. It said they were buried on March 29.

The Associated Press news agency quoted the U.S. Embassy in Baghdad as saying the "international coalition is aimed at Daesh only," using another term for the IS extremist group.

Without directly addressing the Iranian claim, the statement said, "All air strikes are carried out through the alliance with the Iraqi government and in full coordination with the [Iraqi] Ministry of Defense."

The large-scale offensive to retake Tikrit has been waged by Iraqi troops and Shi'ite militias advised by Iranian General Qassem Soleimani, commander of the Guard's elite Quds Force, in early March.

The U.S.-led coalition began air strikes around Tikrit on March 21 to support the operation at the Iraqi government request.

The top U.S. general for the Middle East said on March 26 that Iranian-backed Shi'ite militias have left the fight and pulled back from Tikrit as a condition for U.S. involvement.

Army General Lloyd Austin, head of U.S. Central Command, told the Senate Armed Services Committee he had insisted the militias pull back before the U.S. began flying intelligence-gathering flights and dropping bombs in support of Iraqi soldiers and federal police.

Some militia spokesmen contested that account, saying the forces chose to withdraw to protest U.S. involvement.

Iraq had asked the U.S.-led coalition to mount air strikes on Tikrit after the operation stalled.

Tikrit, the home town of former dictator Saddam Hussain, fell to IS militants in June during an offensive in which they seized much of northern Iraq and Syria.

The U.S.-led coalition began air strikes against the extremist group in August.

Iran has offered advisers and other assistance to Iraq to fight the militants.

Meanwhile, in Baghdad the visiting UN chief Ban Ki-moon held meetings with Iraqi Prime Minister Haidar al-Abadi and other top officials on March 30. Ban also addressed the Iraqi parliament.

Ban's spokesperson said on Twitter that the UN chief would "convey UN support" for the people and government of Iraq "in these challenging times."

No details were provided.

Based on reporting by AP and sepahnews.com