The car had been parked near a restaurant in the northern city, about 175 kilometres (100 miles) north of the capital Baghdad.

Three of the dead were women, the sources said, and another 16 people were wounded.

There was no immediate claim of responsibility for the attack.

Tikrit spent nearly 10 months under Islamic State group rule before Iraqi security forces retook it in the spring of 2015.

Late last year, Iraq announced it had successfully ousted IS from the group's urban strongholds across the country.

But jihadist cells still wage hit-and-run attacks. This month alone, a deadly string of attacks hit the capital and a car bomb exploded in IS's onetime capital Mosul, killing three.