MAIDUGURI, Nigeria (Reuters) - Multiple blasts at camps for people who have fled the Islamist militants Boko Haram killed four and injured 18 in the northeastern Nigeria city of Maiduguri, the state police commissioner said on Wednesday.

The attacks were the latest in a series in the past week. In a video circulated on Friday, a man claiming to be Boko Haram’s leader claimed responsibility for bombings in Maiduguri and a raid in a nearby town last week. Bombings near the city killed four on Sunday.

Borno Police Commissioner Damian Chukwu said the latest blasts happened at around 04:30 a.m. (0330 GMT).

“Three bomb explosions occurred this morning at three different locations near Muna Garage, Maiduguri. Four people were killed and 18 people injured,” he said.

Chukwu said two of the blasts happened at camps for people displaced by Boko Haram. The frequency of attacks has increased since the end of the rainy season in late 2016.

Maiduguri has been the city worst hit by Boko Haram’s eight-year-old campaign to create an Islamic caliphate in northeastern Nigeria. Its attacks have killed about 15,000 people and forced more than 2 million to flee their homes.