ISIS leader Abu Bakr Al Baghdadi was the target of a US special forces raid in north-west Syria on Saturday, according to American media reports.
A senior US military official told CNN that Al Baghdadi was the target, after reports of US military helicopters over Idlib, Syria's last rebel-held bastion. Newsweek and Fox News also reported Al Baghdadi as being the target of the operation, citing military sources.
In a tweet on Sunday morning, US President Donald Trump appeared to allude to the operation, saying “Something very big has just happened!”, without adding any further detail.
Mr Trump is expected to hold a news conference at 9am local time (5pm UAE time) at the White House, deputy press secretary Hogan Gidley announced.
Al Baghdadi may have killed himself with a suicide vest as US special forces descended, American media outlets said, citing several government sources. However, having been pursued for years by the US-led coalition against ISIS, Al Baghdadi has erroneously been reported dead several times.
Officials told ABC News that biometric work was underway to confirm the identification of those killed in the raid.
Syria's top Kurdish commander said he welcomed the "historic operation".
Mazloum Abdi, head of the Kurdish-led Syrian Democratic Forces that was the US's main local ally in years of battles against ISIS in Syria, said the operation was the result of "joint intelligence work".
Turkish and US military authorities exchanged and co-ordinated information before the operation, the Turkish Defence Ministry said on Sunday.
Al Baghdadi has been in hiding for five years and the US had offered a reward of $25 million (Dh91.8m) in return for his capture or death.
He was long thought to hiding somewhere along the Iraq-Syria border. He has led ISIS since 2010, when it was still an underground Al Qaeda offshoot in Iraq.
Reuters reported that Iraq was informed by sources in Syria that Al Baghdadi has been killed, along with his personal body guard after his hiding place was discovered when he tried to get his family out of Idlib towards the Turkish border.
On September 16, ISIS's media network issued a 30-minute audio message purporting to come from Al Baghdadi.
The ISIS chief said operations were taking place daily and called on supporters to free women jailed in camps in Iraq and Syria over their alleged links to his group.
Al Baghdadi also said the US and its proxies had been defeated in Iraq and Afghanistan, and that Washington had been "dragged" into Mali and Niger.
At the height of its power ISIS ruled over millions of people in territory running from northern Syria through towns and villages along the Tigris and Euphrates valleys to the outskirts of the Iraqi capital, Baghdad.
But the fall in 2017 of Mosul and Raqqa, the extremists' strongholds in Iraq and Syria respectively, stripped Al Baghdadi, an Iraqi, of the trappings of a caliph and turned him into a fugitive thought to be moving along the desert border between Iraq and Syria.