The spiral minaret, in the town of Samarra, is more than 1,000 years old.
Police say insurgents blew up the top section of the 52m (162ft) Malwiya tower, which had been used by US soldiers as a lookout position.
The minaret was built by Caliph al-Mutawakil in the 9th century when Samarra, a city on the Tigris north of Baghdad, was capital of the Abbasid
The blast left crumbled brick and clay on the minaret's winding ramps.
Iraq's antiquities officials had expressed concern that US soldiers had also caused significant damage to historic sites in Samarra, including the walls of an ancient palace.
Samarra has been a focal point of the insurgency over the past two years.
The spiral minaret is one of Iraq's main tourist attractions and features on Iraqi banknotes.
The explosion left debris scattered on the minaret's external steps
US troops pulled out of the site last month.
A senior government official told the BBC the Americans should have ensured it was properly protected.
Iraqi antiquities officials have asked for compensation after the walls of an ancient palace in Samarra occupied by the Americans were cracked.
Coalition troops have been heavily criticised for earlier damage done to the ancient site of Babylon which was taken over as a military base.
BBC Baghdad correspondent Caroline Hawley says extensive looting of archaeological sites, particularly in southern Iraq, has also raised serious concerns about the effects of the war on the country's heritage.
Great Mosque of Samarra, Iraq, Izzedine