Suicide bombers in ambulances packed with explosives detonated their vehicles at a checkpoint and a car park for Shi'ite pilgrims in two Iraqi cities on Sunday, killing at least 21 people and wounding dozens, officials told Reuters.
The attacks took place in the towns of Tikrit and Samarra as Iraqi troops, backed by a US-led international coalition intensified their offensive to retake the northern city of Mosul from Islamic State militants who have controlled it for more than two years.
Ultra-hardline Sunni Islamists have struck the Kurdish-controlled city of Kirkuk, the capital Baghdad and a western desert town during the three-week Mosul campaign.
Al-Askari mosque in Samarra, 125 km (80 miles) north of Baghdad is considered one of Shi'ite Islam's holiest shrines.
Reuters reported that ten Iranian pilgrims were among the people killed. Pilgrimage official Mohammad Javad Daneshyar said that the Iranians were getting off a bus when the blast took place.
Authorities in both cities declared curfews, fearing possible further attacks.
An AFP report stated that there was no immediate claim of responsibility for the attacks, but the Islamic State group frequently carries out suicide bombings in Iraq.