More than half of all schools in the northeastern state of Borno, Nigeria, are closed with millions of children unable to start classes this year because of the ongoing threat of Boko Haram, according to the United Nations children's agency.

Almost 1,400 schools have been destroyed in Borno during the armed group's eight-year rebellion, and more than 57 percent of schools are unable to open because of damage or being in areas that remain unsafe, UNICEF said in a statement on Friday.

An estimated three million children are now in need of emergency education support, it said. Nearly 100 children have also been used as "human bombs", so far this year.

"Children in northeast Nigeria are living through so much horror," said Justin Forsyth, deputy director of UNICEF, at the end of a three-day visit to Maiduguri, the epicentre of the crisis in the northeast.

"In addition to devastating malnutrition, violence, and an outbreak of cholera, the attacks on schools is in danger of creating a lost generation of children, threatening their and the country's future," he added.

READ MORE: UNICEF: Boko Haram use of child bombers soars

UNICEF said Boko Haram, whose name roughly translates to "Western education is forbidden", is believed to have killed more than 2,200 teachers since 2009.

Three years ago, the abduction of more than 200 schoolgirls by the armed group in Chibok in northeastern Nigeria sparked global outrage.

Forsyth said nearly one million children had been forced from their homes by the crisis with 450,000 children under the age of five expected to suffer from severe malnutrition this year.

Rights activists say women and children have suffered the most as a result of the violence.

"The conflict has reached to a stage that these insurgents are diverting attention by using children," Hafsat Muhammed, a Nigerian rights activist and journalist, told Al Jazeera last month.

READ MORE: One million forced from school by Boko Haram war

The UN estimates that 20,000 people have been killed and at least 1.7 million displaced since Boko Haram launched its armed campaign in northeast Nigeria eight years ago.

The violence has also spread to neighbouring countries, leading to the closure of more than 2,000 schools in Nigeria, Chad, Niger, and Cameroon, according to a 2015 UNICEF report.