Boko Haram extremists killed 28 people during attacks on remote villages in northeast Nigeria, members of a civilian defense group said on Friday.

A total of 24 people were killed in an attack Tuesday night in Marfunudi, according to resident Abubakar Jojo. The town is roughly 50 miles from the Borno state capital of Maiduguri.

Jojo said the Islamic extremists slit the throats of many of the victims.

Witnesses said the extremists also attacked the village of Kafa on Thursday, killing four people. Sadiq Kaka, a member of a defense group, said the bodies were thrown into a river.

Word of attacks in remote areas often take days to reach population centers.

Meanwhile, the Nigerian army said that during a search operation in villages suspected to be Boko Haram enclaves in Borno state they discovered and destroyed an improvised explosives device-making factory and arrested three suspected Boko Haram members.

More than 1,000 people have been killed since President Muhammadu Buhari was elected in March with a pledge to wipe out the militants, whose six-year-old uprising has killed a total of 20,000 people. Nearly 2 million have been driven from their homes.

Doctors Without Borders said Friday that 75,000 refugees from Niger, Nigeria and Chad have been displaced from their homes in recent weeks due to attacks in the Lake Chad area.

Earlier this year, troops from Chad and Nigeria drove the extremists out of some 25 towns held for months in what had been declared an Islamic caliphate. The insurgents have returned to hit-and-run tactics and suicide bombings.

Hundreds have been freed from Boko Haram captivity, but none of the 219 girls abducted in April 2014 from a school in Chibok were rescued. Thursday marked the girls' 500th day in captivity.

Defense chiefs have finalized details to deploy 8,750 troops from five countries against Boko Haram, Nigeria's Defense Ministry reported Tuesday.