Gunmen killed 15 villagers and abducted five aid workers in separate attacks in Nigeria's troubled northern region, authorities said Thursday.

The assailants arrived in Benue state’s Apa area and opened fire on villagers in their homes, according to David Olofu, a senior state government official. He said military personnel were among those shot in the attack and many houses were razed as villagers fled to safety.

The incident in Benue is the latest in a spiral of violent attacks in which armed groups are targeting remote communities across Nigeria's northwest and central regions, often defying government and security measures.

More than 80 people have been killed in Benue in the past month in such attacks. No group has claimed responsibility for the killings, though authorities have blamed Fulani herdsmen, a group of mostly young pastoralists from the Fulani tribe caught up in Nigeria’s conflict between host communities and herdsmen over limited access to water and land.

In northeastern Nigeria, meanwhile, Islamic extremists abducted five aid workers in Ngala, Borno state, where an insurgency against the government has raged on for more than a decade.

The aid workers included three staff members and two contractors of the international non-government organization FHI 360, all “working to provide lifesaving medical care to the people of Nigeria,” the organization said Thursday, without further details on the incident.

FHI 360 condemned the abduction of the workers and called for their “unconditional, immediate and safe return,” according to a statement from Iorwakwagh Apera, the NGO's director in Nigeria. “Our priority at this time is to support our team and their families,” said Apera.

The Boko Haram extremist group has been waging a bitter war against Nigeria since 2009, and the insurgency has spread over the years to the neighboring countries of Cameroon, Niger and Chad. A breakaway faction of the group formed in 2016 and became known as the Islamic State in West Africa Province and is notorious for targeting security forces and aid workers.