WASHINGTON (AFP) - Islamic State in Iraq and Syria (ISIS) leader Abu Bakr al-Baghdadi has fled Mosul and has apparently delegated tactical control of the battle for the city to local commanders, a US defence official said Wednesday (March 8).

The official said the elusive leader, who appeared in public in Mosul in July 2014 to proclaim a "caliphate," fled the former ISIS bastion some time before Iraqi security forces surrounded the city during an offensive to retake it.

"He was in Mosul at some point before the offensive. We know he's been there," the official told reporters.

"He left before we isolated Mosul and Tal Afar," a town to the west of the city, the official added.

Baghdadi is not believed to be exercising any kind of tactical influence on how the Mosul fight will play out, the official said.

"He probably gave broad strategic guidance and has left it to battlefield commanders."

The hunt for Baghdadi is being led by various groups including US special operations forces, while the anti-ISIS coalition focuses on killing battlefield commanders.

ISIS has lost most of the land it once held in Iraq and Syria but hopes to cling to scraps of a self-declared caliphate, the official said.

Since summer 2014, when ISIS was at its peak just ahead of the US-led war on the group, the extremists have lost 65 per cent of the land they'd seized across much of northern Syria and large parts of Iraq.

ISIS now is looking beyond the seemingly inevitable loss of their strongholds of Mosul in Iraq and Raqa in Syria.

"I don't think they have given up on their vision of their caliphate yet," the official said, noting ISIS hopes to hold on to parts of eastern Syria and western Iraq.

"They still believe they can function and are still making plans to continue to function as a pseudo-state centred in the Euphrates River valley."

In Mosul, Iraqi security forces backed by Western air power have recaptured the eastern side of the city and are making gradual progress into the western side in a bloody fight.

ISIS militants realise their days are numbered in Mosul and, despite having spent two years building defensive measures in Raqa, also understand they will lose that bastion too, the official said.

"Logically, any of those leaders would look at that situation and say from a military perspective this may be not be tenable for us to hold," the official said.

"Raqa would probably not be the final battle against ISIS... There is still ISIS in the rest of the Euphrates river valley downstream that will have to be dealt with."

About 15,000 ISIS fighters remain in Iraq and Syria, including some 2,500 in Mosul and the neighbouring town of Tal Afar and as many as 4,000 still in Raqa, the official said.