Jihadist leader Abu Bakr al-Baghdadi called on his fighters to resist as Iraqi forces were poised to enter the city of Mosul where he declared a "caliphate" two years ago.

"Do not retreat," said a voice presented as belonging to the leader of the ISIS group in an audio message released early Thursday by the ISIS-affiliated Al-Furqan media.

"Holding your ground with honour is a thousand times easier than retreating in shame," he said in the message, his first in more than a year.

"To all the people of Nineveh, especially the fighters, beware of any weakness in facing your enemy," Baghdadi said, referring to the northern Iraqi province of which Mosul is the capital