Scores of female students kidnapped by suspected Islamic militants from a school in the north-east of Nigerian have been freed, Nigeria's military reported on Wednesday.

Only eight of more than 100 students are unaccounted for, major general Chris Olukolade said in a statement that gave no details. "The others have been freed this evening," he said.

The government had reported that security forces were in pursuit of militants who abducted more than 100 females from a high school early on Tuesday.

The Borno state governor, Kashim Shettima, told reporters that 129 students were kidnapped and at least 14 freed themselves. Four of the students – aged between 16 and 18 – jumped off the back of a truck and 10 escaped when their kidnappers asked them to cook and were not paying attention.

The abductions came hours after an explosion blamed on extremists killed 75 people in Nigeria's capital, Abuja, angering citizens who are questioning government and military claims that they are containing a 5-year-old Islamic uprising. Two more attacks killed 20 people on Tuesday night and Wednesday morning in villages in the north-east.

While the military claims that they have cornered insurgents in a remote north-east corner of the west African nation, attacks have increased in frequency and becoming more deadly. More than 1,500 people have been killed this year, compared to an estimated 3,600 between 2010 and 2013.

Shettima told reporters on Wednesday that the insurgents arrived at Chibok government Secondary School for Girls wearing military fatigues and posing as soldiers – a common tactic used by the insurgents. His information came from the school principal, who believed the men were soldiers removing the young women for their own safety. So the principal made no fuss as the students were loaded onto the back of a truck.

It was only as the armed men were leaving and started shooting that he realised his mistake, Shettima said. The militants killed a soldier and a police officer guarding the school, officials said.

Nigeria has Africa's biggest economy but 70% of the population lives below the poverty line and the north-east suffers the most. Only 5% of children get to secondary school and only a small percentage of those are girls. The government closed all schools in Borno three weeks ago. The girls who were kidnapped had been recalled so that they could write their final exams.