"For five months there has been joint intel cooperation on the ground and accurate monitoring, until we achieved a joint operation to kill Abu Bakr al-Baghdadi," Mazloum Abdi said in a post on social media.

In an earlier post, the head of the Kurdish-led Syrian Democratic Forces that was the US's main local ally in years of battles against IS in Syria, described the operation as "historic".

Officials in Washington said a military operation had targeted IS chief Baghdadi overnight in Syria's northwestern province of Idlib, but stopped short of confirming he had been killed.

US President Donald Trump has said he will make a major announcement on Sunday.

The Syria Observatory for Human Rights, a war monitoring group with a vast network of sources on the ground, told AFP that US helicopters had dropped troops on the ground overnight and attacked a target near the village of Barisha in Idlib province.