Militants have killed at least a dozen members of the Iraqi security forces in early morning attacks in the city of Tikrit, according to reports.

Four police officers were shot dead at a checkpoint on the west side of the city. Security forces returned fire and killed one gunman. Other militants continued north to a second checkpoint where they detonated a vehicle rigged with explosives, killing eight more police officers.

The Associated Press put the death toll in the second attack as high as 11. Dozens more were wounded.

Colonel Mohammed al-Jabouri, a spokesman for the police force in Saladin province, said the the local police chief and head of the provincial security committee were visiting the second checkpoint at the time of the attack, but both escaped unharmed.

An unverified photograph shared on social media showed two dark plumes of smoke rising into the sky near arches at the city’s northern gates, Reuters said.

There was no immediate claim of responsibility for the attack, the first of its kind since the city was retaken from Islamic State in April last year.

The governor of Saladin province, Ahmed al-Jabouri, accused Isis of launching the attacks and vowed to “retaliate for the martyrs by chopping off the heads of Daesh militants”, using the Arabic name for the group.

Jabouri called on security forces to review their plans, and on residents to cooperate with the authorities.

Isis militants have previously claimed responsibility for similar attacks targeting Iraq’s security forces and civilians in public areas. The group has stepped up attacks in recent months as it has lost territory in northern and western Iraq that it had captured in 2014.

This week government forces recaptured the town of Shirqat, north of Tikrit. Western officials have indicated that the drive for Mosul, Iraq’s second city and an Isis stronghold, may begin next month, though Baghdad has declined to specify when it will start. Mosul is the last major urban stronghold held by Isis in Iraq.