WASHINGTON (REUTERS) - ISIS leader Abu Bakr al-Baghdadi was believed to have been killed in a US military operation in Syria, sources in Syria, Iraq and Iran said on Sunday (Oct 27), as US President Donald Trump prepared to make a "major statement" at the White House.

A US official, speaking on condition of anonymity, earlier told Reuters Baghdadi was targeted in the overnight raid but was unable to say whether the operation was successful.

A commander of one of the militant factions in the northwestern Syrian province of Idlib said Baghdadi was believed to have been killed in a raid after midnight on Saturday involving helicopters, warplanes and a ground clash in the village of Brisha near the Turkish border.

Two Iraqi security sources and two Iranian officials said they had received confirmation from inside Syria that Baghdadi had been killed. "Iran was informed about Baghdadi's death by Syrian officials who got it from the field," one of the officials said.

Iraqi state television on Sunday aired footage of what it said was the US raid that killed Baghdadi.

Day-time footage showed a crater in the ground and what appeared to be the aftermath of a raid, with torn blood-stained clothes on the ground. It also showed night-time footage of an explosion.

The broadcaster quoted an expert on terrorism saying that Iraqi intelligence agencies had helped pinpoint Baghdadi's location.

Newsweek, which first reported the news, said it had been told by a US Army official briefed on the raid that Baghdadi was dead. It said the operation was carried out by special operations forces after receiving actionable intelligence.

The Pentagon did not immediately respond to a request for comment.

White House spokesman Hogan Gidley announced late on Saturday that Trump would make a "major statement" at 9am local time (9pm Singapore time) on Sunday. Gidley gave no further details as to the topic of Trump's statement.

The president gave an indication that something was afoot earlier on Saturday night when he tweeted without explanation,"Something very big has just happened!"

The New York Times said US Special Operations commandos carried out the raid against a senior terrorist leader, quoting two senior administration officials.

A senior US official said commandos and analysts were still seeking to confirm the identity of the terrorist, who the officials said was killed when he exploded his suicide vest.

A person close to Trump said that the target of the raid was Baghdadi.

Officials said the raid was in Idlib province, hundreds of kilometres from the area where Baghdadi was long believed to be hiding along the Syrian-Iraqi border. Idlib is dominated by jihadi rebel groups hostile to him.

A US official told the New York Times that commandos from the Army's elite Delta Force carried out the mission with the CIA providing intelligence and reconnaissance information on the ground.

Trump has faced withering criticism from both Republicans and Democrats alike for his US troop withdrawal from northeastern Syria, which permitted Turkey to attack America's Kurdish allies.

Many critics of Trump's Syria pullout have expressed worries that it would lead that the Islamic State in Iraq and Syria (ISIS) militant group to regain strength and pose a threat to US interests. An announcement about Baghdadi's death could help blunt those concerns.

Trump was expected to make the statement in the White House Diplomatic Reception Room, which he has used to make a number of major announcements. Just last week he used the same room to announce that a ceasefire between Turkey and the Kurds had taken hold.

For days, US officials had feared that ISIS would seek to capitalise on the upheaval in Syria.

But they also saw a potential opportunity, in which ISIS leaders might break from more secretive routines to communicate with operatives, potentially creating a chance for the United States and its allies to detect them.

The United States had put up a US$25 million (S$34 million) reward for the capture of Baghdadi, who has led the group since 2010, when it was still an underground offshoot of al Qaeda in Iraq..

On Sept 16, ISIS' media network issued a 30-minute audio message purporting to come from Baghdadi, in which he 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.

In the audio message, Baghdadi also said the United States and its proxies had been defeated in Iraq and Afghanistan, and that the United States 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, its strongholds in Iraq and Syria respectively, stripped 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.

US air strikes killed most of his top lieutenants, and before ISIS published a video message of Baghdadi in April there had been conflicting reports over whether he was alive.

Despite losing its last significant territory, ISIS is believed to have sleeper cells around the world, and some fighters operate from the shadows in Syria's desert and Iraq's cities.