"Do not relent against your enemy... The battle is not yet raging, but it will rage in Baghdad and Karbala," Adnani said, according to a SITE translation of an audio statement released on the militants' Twitter feed.
"Put on your belts and get ready."
Adnani also dismissed Prime Minister Nouri al-Maliki as “woefully incompetent.”
"What do you know about policy, leadership, and military command? You lost a historic opportunity for your people to control Iraq ... indeed, there is between us and you a balance to be made even,” the terrorists spokesman said in the translated statement.
Earlier on Wednesday, Maliki said that the seizure of Mosul was a conspiracy, vowing that the government will hold perpetrators responsible, AFP reported.
In a televised speech, Maliki said he will form an army of volunteers to support the regular armed forces in the face of the terrorist attack.