TIKRIT, Iraq (Reuters) - Eighteen people, including two children, were killed on Saturday when militants shot at a checkpoint north of Tikrit and then set off two bombs at the entrance to the Iraqi city in attacks later claimed by Islamic State.

It was the first such assault since Tikrit, 150 km (95 miles) north of Baghdad, was retaken from the ultra-hardline jihadist group in April 2015.

One militant was killed at the checkpoint after shooting dead five security personnel and two young civilian bystanders at around 5 a.m., according to police and sources from Salahuddin Operations Command, which is responsible for security in the area.

Two other militants continued on about 7 km to the city limits where one of them detonated the explosives in his pickup truck and the other set off an explosives-filled vest, the sources said. The blasts killed 11 people, including three members of a Shi’ite militia working alongside the security forces, and wounded 21.

Amaq news agency, which supports Islamic State, said the group was responsible for the violence. A statement circulated online by supporters said four militants had perpetrated the attacks, which had targeted police and “rejectionist Hashid”, a the Sunni militants’ term for Shi’ite militiamen.

Police tightened security following the attacks in the city, which already had some of the most restrictive measures in place following its recapture from Islamic State.

The city and surrounding areas have become a major hub for local and international aid groups providing assistance to civilians forced from their homes by fighting between Islamic State and Iraqi forces further north.

More than 3.3 million Iraqis have been displaced since early 2014 and up to 1.5 million more could be affected in a push on the Islamic State stronghold of Mosul later this year, according to the United Nations.

The military this week retook the district of Shirqat, 100 km north of Tikrit, from Islamic State in preparation for the move on Mosul.