MAIDUGURI, Nigeria (Reuters) - Four female teenage suicide bombers killed two people and injured 16 others in a residential area in the northeast Nigerian city of Maiduguri, a disaster agency spokesman said on Wednesday.

The girls knocked on the door of a house and then detonated their devices, a representative of the state-run emergency service in Borno State said.

The focus on individual homes is a new tactic.

“Community leaders should create awareness among residents not to open their doors for anybody (if) they are not aware of the visit,” Borno police commissioner Damian Chukwu said.

The blasts in the Muna Garage area, on the edge of the city worst hit by jihadist group Boko Haram’s eight-year insurgency, occurred around 1:15 a.m (0015 GMT), National Emergency Management Agency (NEMA) spokesman Abdulkadir Ibrahim said.

“Four female teenage suicide bombers and two other men died,” he said.

Hours later, suspected Boko Haram militants attacked the town of Magumeri, around 50 kilometers (31 miles) from Maiduguri, shooting indiscriminately and forcing locals to flee their homes, witnesses said.

Residents said the attackers burned down buildings and opened fire after arriving in vans and on motorcycles at around 05:00 p.m. (1600 GMT).

“They were shooting sporadically, one person was dead near them. The police station, village head’s house and other residences were burnt down,” civil servant Mustapha Aja said by telephone. He said he had been separated from his wife and children.

The number of attacks or attempted attacks bearing the hallmarks of Boko Haram in crowded areas, such as markets and refugee camps, has escalated since the end of the rainy season in late 2016.

Most of the attacks have either been foiled or the suicide bombers have only managed to blow themselves up.

The jihadist group has killed 15,000 people and displaced more than two million during its campaign to create an Islamic state governed by a harsh interpretation of sharia law in the northeast of Africa’s most populous nation.

It has also carried out cross-border attacks in neighboring Cameroon, Chad and Niger.

Cameroon said on Wednesday that West African forces had freed 5,000 people being held in villages by Boko Haram, in an operation that killed more than 60 fighters and destroyed the jihadist group’s hideout along the Nigeria-Cameroon border.