WASHINGTON (AFP) - The United States on Friday (Dec 16) more than doubled the bounty on the head of the shadowy leader of the Islamic State in Iraq and Syria (ISIS) group, Abu Bakr al-Baghdadi, to US$25 million (S$36 million).

The announcement by the State Department "Rewards for Justice Program" came as US-backed local forces close in on the militant movement's main urban strongholds in Iraq and Syria, the cities of Mosul and Raqa.

The cash will be paid to anyone who can offer "information leading to the location, arrest or conviction" of the elusive militant, known to his followers as "Caliph Ibrahim".

"Under al-Baghdadi, ISIL has been responsible for the deaths of thousands of civilians in the Middle East, including the brutal murder of numerous civilian hostages from Japan, the United Kingdom, and the United States," the State Department said, using an alternative acronym for ISIS.

"The group also has conducted chemical weapons attacks in Iraq and Syria in defiance of the longstanding global norm against the use of these appalling weapons, and has enabled or directed terrorist attacks beyond the borders of its self-declared caliphate."

Baghdadi has kept a low profile, despite having declared himself the leader of a renewed Muslim caliphate, but last month released a defiant audio message urging his supporters to defend Mosul.

It is not clear if he is in the besieged city, where he declared his caliphate in 2014 after the ISIS group seized territory covering much of eastern Syria and northern Iraq.

The video, which showed a man with a black and grey beard wearing a black robe and matching turban, is the only one ISIS has released of Baghdadi to date.

He has been reported wounded in US-led coalition air strikes multiple times, but the claims have never been verified, and his apparent survival has added to his mystique.

According to an official Iraqi government document, Baghdadi was born in Samarra in 1971 and has four children with his first wife - two boys and two girls born between 2000 and 2008.

An Iraqi intelligence report records that Baghdadi has a PhD in Islamic studies and was a professor at Tikrit University.

Baghdadi apparently joined the insurgency that erupted after the 2003 US-led invasion of Iraq and spent time in an American military prison.