A car bomb targeting a military checkpoint on a road leading to Nigeria's northeastern city of Maiduguri exploded Tuesday, killing all nine people in the vehicle, police and witnesses said.

It was unclear if any soldiers were hurt in the attack, which was blamed on Boko Haram extremists.

Preliminary reports indicate that those killed were the driver and passengers in the explosives-laden minivan, said police deputy superintendent Victor Isuku.

The van came off a rural road and blew up shortly after turning onto the tarred main road near Gubio town in a "massive blast," said truck driver Habib Isa. Gubio is 60 miles northeast of Maiduguri, the birthplace of Boko Haram and headquarters of the military campaign to halt the insurgency.

Boko Haram recently has stepped up attacks after a months-long lull caused by a leadership struggle in the Islamic State-affiliated group.

Tuesday's explosion follows four suicide bombings in three weeks in Maiduguri, which is home to nearly as many refugees from Boko Haram as residents. Two of those suicide bombers tried, but failed, to enter a camp housing more than 16,000 refugees.

The uprising has killed more than 20,000 people over seven years and forced some 2.6 million people from their homes.