"Do not retreat," said a voice presented as belonging to the leader of the Islamic State group in an audio message released early Thursday by the IS-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.
Rumours have abounded about the Iraqi jihadist leader's health and movements but his whereabouts are unclear.
In June 2014, days after jihadist fighters swept across swathes of Iraq, he made a rare public appearance in Mosul and announced the creation of an Islamic "state" straddling Iraq and Syria.
The "caliphate" has been shrinking steadily since last year and Iraqi forces earlier this week reached Mosul, the jihadists' last major stronghold in Iraq.
The US-led coalition estimates there are 3000 to 5000 IS fighters inside the city but the final outcome of the battle appears to be in little doubt.
Tens of thousands of Iraqi forces, backed by the US-led coalition and its warplanes, launched a massive offensive on Mosul on October 17.
