MAIDUGURI, Nigeria (Reuters) - Five female suicide bombers killed 12 people and wounded 11 in northeast Nigeria’s Borno state, birthplace of the Islamist militant Boko Haram insurgency, police said on Monday.

There was no immediate claim of responsibility for the attacks. But the use of female suicide bombers in public places is a tactic used by Boko Haram, which has focused on Borno during its eight-year-old insurgency aimed at creating an Islamic caliphate.

Borno state police spokesman Victor Isuku said the attacks occurred on Sunday at around 08:30 p.m. (3.30 a.m. ET) in the village of Kofa, 8 km (5 miles) from the state capital Maiduguri.

“The first suicide bomber detonated near a mosque, killing seven persons. The second detonated in a house killing five persons,” he said, adding that 11 people hurt in the attacks. Isuku said the other three bombers also died.

Borno has been the state worst hit by the insurgency that has killed more than 20,000 people and forced some 2.7 million people to flee their homes since 2009.

Boko Haram killed 14 people in bombings and shootings in Maiduguri on June 7.