Iraq claimed Sunday to have struck a convoy carrying ISIS group leader Abu Bakr al-Baghdadi in an air raid near the Syrian border but said his fate was unknown.
In Syria meanwhile regime troops backed by Russian air strikes made progress on two fronts but were battling other rebel groups.
Iraqi aircraft struck Baghdadi's convoy as it was "moving towards Karabla to attend a meeting of the Daesh (ISIS) terrorist leaders," an Iraqi security statement said.
Karabla lies on the Euphrates barely five kilometres (three miles) from the Syrian border in western Anbar, a vast Iraqi province which has long been a Sunni insurgent stronghold.
The statement issued by Iraq's "war media cell" said Baghdadi was "transported in a vehicle" after the strike but added that "his health status was unknown".
The meeting place was also struck in the operation and several ISIS leaders already gathered there were killed and wounded, it said.
Interior ministry spokesman Saad Maan told AFP that "the strike was yesterday (Saturday) at noon."
In Washington, a US military official said: "We've seen the Iraqi statement about al-Baghdadi but have no info that confirms it."
Baghdadi's death would give the war against the most violent militant organisation in modern history a much-needed boost but Iraqi security sources have made such claims in the past.
"It's hard to confirm, there is definitely a psychological war going on between Iraqi intelligence and ISIS," Iraqi analyst Ihsan al-Shammari said.
"But even if they just hit his convoy, that would show a real improvement in the field of intelligence," he said.
Baghdadi's apparent survival following similar claims, including one in November 2014 of a strike in the same area, has only added to his mystique.
The Iraqi insurgent chief is said to have been born in the city of Samarra in 1971 but little is known about the man, who has a $10 million US bounty on his head.
Baghdadi apparently joined the insurgency that erupted after the 2003 US-led invasion of Iraq, at one point spending time in an American military prison in the country's south.
His whereabouts have been the subject of constant speculation since his only public appearance as ISIS boss last year, at a mosque in the Iraqi city of Mosul days after the proclamation of a cross-border "caliphate".
A coalition led by the United States, which occupied Iraq for eight years before withdrawing in 2011, began air strikes against IS in August last year.
More than 7,000 strikes later, ISIS has conceded some of the territory it took last year but has held its ground in other areas and even made fresh conquests, such as the city of Ramadi in Iraq and Palmyra in Syria.
Russia joined the fray on September 30, launching an air campaign it said would target ISIS, but most of its efforts seem focused on protecting Syrian President Bashar al-Assad.
Syrian troops Sunday were gaining ground on two fronts: in central Hama province around the Damascus-Aleppo highway and in the northern part of regime stronghold Latakia province.
In Hama, regime forces had taken three villages east of the highway and were seeking to also secure control of an area to its west.
"This offensive is intended to confront the rebels in the Sahl al-Ghab plain that is at the intersection of Hama, Latakia and Idlib provinces," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights monitor.
Alawites from the same sect as Assad live in the south and west of the strategic 1,000 square kilometre (385 square mile) plain, while the north and east are mostly Sunni.
In recent months, rebels have sought to capture parts of the area, advancing particularly from Idlib province, which is held by the Army of Conquest alliance that includes Al-Qaeda affiliate Al-Nusra Front.
In Moscow, Russia's defence ministry said Sunday that its fighter jets had hit 63 targets in Syria in the last 24 hours and reported it had intercepted radio traffic showing "growing panic" among IS fighters.
Human Rights Watch meanwhile on Sunday accused Russia of being behind the use of new advanced cluster munitions in Syria.
The New York-based group said it had obtained photographs showing cluster munitions were dropped on Kafr Halab, a village southwest of Syria's second city of Aleppo, on October 4.
It said it could not confirm whether the munitions had been used by Russian forces, or supplied by Moscow but used by Syria.
"It's disturbing that yet another type of cluster munitions is being used in Syria given the harm they cause to civilians for years to come," said Nadim Houry, HRW's deputy Middle East director.