As Nigeria prepares to mark the fourth anniversary of the Chibok kidnapping, Unicef reported on Friday that more than 1,000 children have been abducted by jihadists since 2013.

"Since 2013, more than 1,000 children have been abducted by Boko Haram in northeastern Nigeria, including 276 girls taken from their secondary school in the town of Chibok in 2014," said the United Nations children's fund in a statement.

"These repeated attacks against children in schools are unconscionable," said Mohamed Malick Fall, a Unicef representative in Nigeria.

Boko Haram's fight to establish a hardline Islamic state in northeast Nigeria has claimed at least 20,000 lives and displaced more than two million people.

Schools, particularly those with a secular curriculum, have been targeted by Boko Haram, whose name roughly translates from Hausa - the language spoken widely across northern Nigeria - as "Western education is forbidden".