MOSCOW (AFP, REUTERS) - The Russian army on Friday (June 16) said it hit ISIS leaders in an airstrike in Syria last month and was seeking to verify whether the militant group's chief Abu Bakr al-Baghdadi had been killed.

The United States said it could not immediately confirm Baghdadi's death.

In a statement, the Russian army said Sukhoi warplanes carried out a 10-minute night-time strike on May 28 at a location near Raqa, where Islamic State in Iraq and Syria (ISIS) leaders had gathered to plan a pullout by militants from the group's stronghold.

"Senior commanders of the military groups of the so-called IS military council, 30 mid-ranking field commanders and up to 300 militants who provided security for them were eliminated," it said. ISIS is also known as IS and ISIL.

"According to information which is being checked through various channels, the leader of ISIL Ibrahim Abu-Bakr al-Baghdadi was also present at the meeting and was eliminated by the strike," it said.

The US-led coalition battling the group said on Friday it could not confirm the report. "We cannot confirm these reports at this time," said US Army Colonel Ryan S. Dillon, spokesman for the coalition's Operation Inherent Resolve in Syria and Iraq.

RIA news agency quoted Russian Foreign Minister Sergei Lavrov as saying on Friday Russia does not have 100 per cent confirmation that Baghdadi was killed.

OPERATING CAUTIOUSLY

A colonel with the Iraqi national security service told Reuters Baghdadi was not believed to have been in Raqqa at the time of the strike in late May. One of Baghdadi's aides may have been killed rather than Baghdadi himself, the colonel said.

He said that Baghdadi was believed to be operating cautiously in the border area between Iraq and Syria with just a handful of close aides, and avoiding using telecommunications equipment to evade surveillance.

Another Iraqi intelligence official said the Russians had not shared any information with Iraqi authorities to indicate Baghdadi was killed. Iraq was checking the report and would announce his death if it received "solid confirmation".

Hoshiyar Zebari, a long-serving former Iraqi foreign minister and now a senior adviser to the government of the Kurdish autonomous region, also told Reuters there was no confirmation of Baghdadi's death.

Baghdadi's group has earned global notoriety for imposing a hardline form of Islam that has included stonings, beheadings and amputations.

The Iraqi-born world's most-wanted man has been rumoured wounded or killed a number of times in the past.

He has been nicknamed "The Ghost" as he has been reportedly spotted around the Syrian-Iraqi border but his whereabouts have never been confirmed.

CLOSE TO DEFEAT

ISIS fighters are close to defeat in the twin capitals of the group's territory, Mosul in Iraq and Raqqa in Syria, after nearly three years ruling over millions of people in a wide swathe of territory in both countries.

Russia supports the Syrian government of President Bashar al-Assad, which is fighting against ISIS fighters, one front in a multi-sided civil war. The US supports Kurdish and Arab fighters in Syria who are separately planning an assault on Raqqa.

In Iraq, the US-backed government has been battling to recapture Mosul since October last year after driving the group out of most of the rest of the territory it had seized.

The last public video footage of Baghdadi shows him dressed in black clerical robes declaring his caliphate from the pulpit of Mosul's mediaeval Grand al-Nuri mosque back in 2014.

Born Ibrahim al-Samarrai, Baghdadi is an Iraqi in his mid-forties, who broke away from Al-Qaeda in 2013 after years participating in the insurgency against US forces in Iraq and the Iraqi government. The US State Department has offered a US$25 million (S$34.6 million) reward for information leading to his arrest.

A number of senior ISIS figures have been killed in air strikes or special forces raids since the US launched its campaign against the group in 2014, including Baghdadi's deputy Abu Ali al-Anbari, the group's "minister of war" Abu Omar al-Shishani, and its media director Abu Muhammad al-Furqan.