Iran Claims a US Drone Killed Two Revolutionary Guard Members Fighting the Islamic State in Iraq

Iran's Revolutionary Guard has claimed that two of its advisers involved in a campaign to retake the Iraqi city of Tikrit from the Islamic State were killed by an American drone strike — an allegation the US rejected out of hand.

The Tikrit campaign has featured heavy involvement from Iranian personnel and advisers alongside Iranian-backed Shia militias and Iraqi security forces. After initially shying away from involvement, the US began bombing Islamic State (IS) targets Wednesday night as the campaign appeared to stall.

Iranian authorities said on a Revolutionary Guard website that the two advisors — identified as Ali Yazdani and Hadi Jafari — were killed March 23 and buried Sunday.

The US Central Command (Centcom) said in a statement to VICE News that the timeframe given by the Iranians made it impossible for the deaths to be traced to US munitions.

Related: Iraqi forces advance in the battle for Tikrit, with a little help from Iran 

"Coalition forces initiated airstrikes near Tikrit on March 25, two days after the alleged incident occurred and no airstrikes were conducted in or near Tikrit on March 23," the Centcom statement said.

"We have no information to corroborate claims that coalition airstrikes killed two IGRC members," the statement added, using an acronym for the Revolutionary Guard.

Centcom also confirmed that US reconnaissance operations around Tikrit began March 21, two days before the alleged incident. It's unclear whether armed drones were involved in those operations.

US officials have expressed alarm at the extent of Iranian involvement in Iraqi operations to recapture territory seized by IS last summer, when the country's security forces seemingly melted away from northern and western Iraq. Qasem Suleimani, the commander of Iran's Quds Force, has taken a prominent role in organizing the Tikrit campaign. During the US occupation of Iraq, Washington linked Suleimani's Quds Force to shipments of explosive devices that Shia militias used to kill hundreds of American soldiers.

In addition to coordinating military maneuvers in Iraq, Suleimani, once seen as a reclusive figure, has repeatedly appeared on social media in pictures purportedly taken near the frontline of battles with IS.

Related: The shady past of the Iranian general leading the fight against the Islamic State 

Despite US insistence that Iranian-backed Shia militias had pulled back from the fight in Tikrit, reports indicate they are still taking part. In a video posted by Asaib al Haq — one of the militias closest to Iran — the group said it would maintain a presence in Tikrit.

On Friday, Iraqi media reported that American bombs struck Iraqi positions near the University of Tikrit. The US Embassy in Baghdad denied those reports to the Associated Press.

American bombs continued to fall Sunday night in Tikrit, with three airstrikes targeting what US officials said were IS buildings and armored vehicles.

Iraqi officials told the New York Times that as few as 400 IS holdouts remain in Tikrit, part of a group that has repelled some 30,000 Iraqi troops and militia members for four weeks. The inability of Iraqi forces to capture the city despite their overwhelming manpower advantage was reportedly what prompted the US to lend its reluctant support to the campaign.

American involvement in Tikrit comes amid meetings between Iranian and Western negotiators in Lausanne, Switzerland to hash out a last-minute deal on Iran's nuclear program before a self-imposed deadline that expires Tuesday.

Adding to the precarious regional dynamics, a Saudi-led and American-supported air campaign to repel the advance of Houthi rebels in Yemen is now in its fourth day. The intervention in Yemen has drawn the ire of Iran, which is suspected of supporting the Shia rebel group.

UN Secretary General Ban Ki Moon, speaking to reporters in Baghdad during a one-day trip to Iraq, said he was concerned about reports of atrocities committed by Iraq's Shia militias during their campaign against IS.

Related: The offensive to retake Tikrit from the Islamic State has stalled — but it's not over yet 

Ban said he was troubled by "allegations of summary killings, abductions and destruction of property perpetrated by forces and militias fighting alongside Iraqi armed forces."

"Civilians freed from the brutality of Daesh should not have to then fear their liberators," Ban said, using an acronym that spells out the Islamic State's full Arabic name.

Earlier in March, UN investigators released a damning and extensive report that documented atrocities committed by IS in Iraq. The UN also found that both Iraqi security forces and Shia militias may have committed war crimes, an allegation backed by numerous reports from human rights groups.

Last week, the UN's human rights council adopted a resolution denouncing the Islamic State for its actions. Human Rights Watch criticized the UN statement for failing to also condemn the actions of the Iraqi government and militias. In a statement, Human Rights Watch warned further military operations could risk "placing large numbers of civilians at risk."

Following the retreat of IS militants in Amerli, a town in northern Iraq, last September, Human Rights Watch found that that Shia militias looted and burned Sunni homes, destroying "two entire villages."

Follow Samuel Oakford on Twitter: @samueloakford