About 30 mid-level militant leaders and 300 other fighters may have also been killed, says the military.

The Russian army on Friday said it was seeking to verify whether Islamic State chief Abu Bakr al-Baghdadi was killed after its warplanes hit the group's leaders in a night raid in Syria last month.

In a statement, the army said Sukhoi warplanes carried out a 10-minute night-time strike early on May 28 at a location near Raqqa, where IS 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.

"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.

ISIL is an acronym for the so-called IS group, also known as ISIS and Daesh.

The strike took place from 2135 to 2145 GMT on May 27 following the confirmation of the meeting by a drone. Those killed included the emir of Raqa and the IS security chief, the statement said.

The U.S. has been informed about the attack, the statement added.

Baghdadi has not been seen in public since proclaiming himself "caliph" in the Iraqi city of Mosul three years ago.

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

'Matter of time'

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.

In March, U.S. Secretary of State Rex Tillerson declared that Baghdadi's death is imminent as "nearly all" of his deputies are dead and "it is only a matter of time" before the IS chief himself met the same fate.

In September 2015, Russia launched a bombing campaign in Syria in support of Syrian President Bashar al-Assad and its efforts against IS jihadists are separate from the US-led coalition, which supports Syrian rebels.

The Russian military said earlier this month that its planes also struck IS convoys attempting to leave Raqqa from the south toward Palmyra on May 29, killing 80 militants.

On May 31, Moscow also struck IS contingents that had left Raqqa and were located near Palmyra by firing guided missiles from warships deployed off the Syrian coast.

Meanwhile, U.S.-backed Syrian Democratic Forces have been pushing into Raqqa from the north, last Tuesday breaking into the IS bastion for the first time since it became a hub of the group's self-declared caliphate and the scene of its most gruesome atrocities.

Iraqi forces are also battling IS in the group's second hub Mosul, where the jihadists are confined to just a few neighbourhoods in the west of the city.

The Raqqa advance caused alarm this week at the United Nations, whose investigators said civilian loss of life was "staggering" due to "excessive" air strikes.