ISIS leader Abu Bakr al-Baghdadi is believed to have been killed by the US-led forces in Syria.

A senior US defence official has said he was killed in a raid in northwest Syria on Saturday, with the CIA assisting in locating the ISIS target, according to CNN.

US President Donald Trump posted a cryptic tweet on Saturday (local time), alluding to the suspected death.

He posted the tweet that hinted at some "very big" news by saying "something very big has just happened!"

White House Spokesman Hogan Gidley said the announcement will take place at 9 am on Sunday (midnight AEDT).

Trump has been under pressure over a Democratic-led impeachment inquiry and has faced stiff criticism for his US troop withdrawal from Syria.

Baghdadi, the leader of the terror group, has been in hiding for the last five years.

In April, a video was published by the ISIS media wing al-Furqan that showed a man purporting to be Baghdadi.

It was the first time Baghdadi has been seen since July 2014, when he spoke at the Great Mosque in Mosul.

In February 2018, several US officials said Baghdadi had been wounded in an airstrike in May 2017 and had to relinquish control of the terror group for up to five months because of his injuries.

Baghdadi became the leader of the Islamic State of Iraq (ISI) in 2010.

In 2013, ISI declared its absorption of an al Qaeda-backed militant group in Syria and Baghdadi said that his group will now be known as Islamic State in Iraq and the Levant (ISIL or ISIS).