Islamic State leader Abu Bakr al Baghdadi is believed to have been killed in a US military operation.

Al Baghdadi, who had a $25m (£19.5m) bounty on his head, has been the subject of an international manhunt for years but there is no official confirmation as to whether the operation was successful.

Newsweek said it had been told by a US Army official that al Baghdadi was dead, as a result of the military raid in Syria's northwestern Idlib province.

He was known to be at the location of the operation 48 hours prior to the attack, a senior Turkish official said.

An official told the Associated Press that confirmation of the IS boss's death in an explosion is pending.

The Pentagon has not commented.

US President Donald Trump tweeted on Saturday night: "Something very big has just happened!" - but he gave no further explanation.

White House spokesman Hogan Gidley said the president would make a "major statement" at 1pm GMT on Sunday but he did not say what it would be about.

The senior Turkish official, who has not been named, said: "To the best of my knowledge, Abu Bakr al-Baghdadi arrived at this location 48 hours prior to the raid. We have been in close coordination with the relevant parties."

The Syrian Observatory for Human Rights, a Syria war monitor, reported an attack carried out by a squadron of eight helicopters accompanied by a warplane belonging to the international coalition.

IS operatives are believed to be hiding in the area, it said.

It said the helicopters targeted IS positions with heavy strikes for about two hours, during which jihadists fired at the aircraft with heavy weapons.

The UK-based observatory, which operates through a network of activists on the ground, documented the deaths of nine people as a result of assault.

It is not yet known whether al Baghdadi is one of them, it said, adding that the death toll is likely to rise due to the large number of wounded.

A senior Iraqi security official, who asked for anonymity, told the Associated Press that Iraqi intelligence played a part in the operation.

He said al Baghdadi and his wife both detonated explosive vests they were wearing during the US commando operation, and said other IS leaders were killed in the attack.

The strike came amid concerns that a recent US pullback from northeastern Syria could infuse new strength into the militant group, which had lost vast stretches of territory it had once controlled.

Al Baghdadi, who led IS for the past five years, was seen in the summer of 2014 in the pulpit of the Nouri mosque in Mosul.

He urged Muslims around the world to swear allegiance to the caliphate and obey him as its leader.

"It is a burden to accept this responsibility to be in charge of you," he said in the video.

"I am not better than you or more virtuous than you. If you see me on the right path, help me. If you see me on the wrong path, advise me and halt me. And obey me as far as I obey God."

In 2015, he was reportedly severely injured in an airstrike in western Iraq, and he was possibly even killed a few years later according to Russian officials.

He has also been heard in a number of audio messages to followers, including an 18-minute speech given earlier this year.

Al Baghdadi oversaw a shift away from large-scale attacks towards smaller-acts of violence that would be harder for law enforcement to prevent.

He encouraged jihadists who could not travel to the caliphate to kill where they were, with whatever weapon they had at their disposal.

In the US, multiple extremists have pledged their allegiance to al Baghdadi on social media, including a woman who along with her husband committed a 2015 massacre at a holiday party in San Bernardino, California.