After a nearly yearlong silence, Abu Bakr al-Baghdadi, the self-declared caliph of the Islamic State, released a blistering audio recording imploring his forces to remain firm in the face of the American-backed Iraqi offensive in Mosul and excoriating those who might consider fleeing.

“Know that the value of staying on your land with honor is a thousand times better than the price of retreating with shame,” he said, adding: “This war is yours. Turn the dark night of the infidels into day, destroy their homes and make rivers of their blood.”

The last time Mr. Baghdadi addressed his followers was in a recording released Dec. 26. His silence since then has led to persistent rumors that he had been wounded or killed. He was not heard from even after one of his closest associates — the extremist organization’s spokesman, Abu Muhammad al-Adnani, who headed the group’s efforts to export terror abroad, including overseeing attacks in Paris and Brussels — was killed in an airstrike in August.

The terrorist leader’s tone in the new recording at times suggested an air of panic, as if he was trying to shore up his fighters and enjoin them to continue battle, promising them heavenly rewards: “Oh soldiers of the caliphate, if you stand in the line of fire from America’s jets and its allies, then stand firm.”