Islamic State militants drove a tank rigged with explosives into a base south of the Iraqi city of Samarra on Monday, killing 38 policemen, military and police sources said.
A further 46 people were wounded in the attack on the Muthanna bases in an area from which security forces and Shi'ite paramilitaries have been fighting to drive out the insurgents.
Security forces seized the complex from the militants several days ago and were using it as a foothold in an offensive aimed at cutting Islamic State supply lines from the Samarra area to the western province of Anbar.
Anbar's capital, Ramadi, was overrun by the insurgents two weeks ago in the most significant setback for Iraqi forces since a US-led coalition began bombing the militants last summer.
Iraqi Prime Minister Haider al-Abadi vowed to launch a quick counter-offensive to retake the city but Shi'ite paramilitary groups have taken charge of the offensive and say remaining militant strongholds in Salahuddin province must be cleared first.