At least 50 Boko Haram fighters were killed on Tuesday and seven members of Nigeria's infrastructure security force were missing following an insurgent ambush on a convoy monitoring the country's power grid installations, a spokesperson said.

Boko Haram, which has waged an insurgency for 15 years mainly in the northeast, has been weakened by the military and internal fighting but remains a threat as it makes deadly attacks against civilians and government targets.

Babawale Afolabi, spokesperson for the Nigerian Civil Defence Corp, a government agency set up to protect infrastructure, said security operatives were ambushed by about 200 Boko Haram fighters during the patrol mission.

Afolabi said more than 50 insurgents were killed in the fighting but seven operatives were missing, adding that efforts are underway to find them in the bush. He said "a few others" of the security force were wounded.

Although Boko Haram mainly operates in the northeast, Nigerian authorities say the group has cells in the largely Muslim Niger state, where they have previously carried attacks against the military and civilians.

In a separate attack in northeast Borno state, a military spokesperson said five soldiers were killed by suspected insurgents last Saturday.

Reuters