A security source said the blasts rocked the northern entrance of Tikrit, which is the capital of Salahuddin Province, on Saturday.
Bombs loaded on a car have set off at the Aqwas checking point killing scores of people at the entrance to Tikrit.
This was followed by two suicide bombers who detonated their explosive belts.
At least nine police officers and three volunteer fighters of the Popular Mobilization Units were killed, and injuring a further 23 others.
The fatalities included nine police officers and three members of the Iraqi volunteer fighters known as al-Hashad al-Sha’bi or Popular Mobilization Units, the al-Forat news agency said.
Tikrit was overrun by ISIS in June 2014, but was liberated only a year later.