The twin attacks took place in Tikrit and Samarra, as Iraqi troops and security forces battled to retake the northern city of Mosul from Daesh.
Suicide bombers driving ambulances packed with explosives detonated their vehicles at a checkpoint and a car park for Shia pilgrims in two Iraqi cities on Sunday, killing at least 21 people and wounding dozens, police officials and medics said.
The twin attacks took place in Tikrit and Samarra, as Iraqi troops and security forces battled to retake the northern city of Mosul from the Daesh terrorist organisation.
In Tikrit, a bomber detonated his explosives-laden ambulance at the southern entrance to the city during the morning rush hour, killing 13 people, police and hospital sources said.
Another attacker detonated a vehicle in a car park for pilgrims visiting one of Shia Islam's holiest shrines, al-Askari mosque in Samarra, south of Tikrit.
The bomb killed at least eight people, local officials said, including two Iranian pilgrims. The local operation command, a joint military and police unit, said the vehicle used in Samarra was also an ambulance.
Authorities in both cities declared curfews, fearing possible further attacks.
Iraqi troops and security forces, backed by a US-led international coalition, are battling Daesh in the northern city of Mosul. Special forces have entered eastern districts, where they are facing fierce resistance from Daesh, which has deployed car bombs, snipers and mortar fire against them.