Nobel Peace Prize laureate Malala Yousafzai has criticised Nigerian leaders for not doing enough to find the 219 missing schoolgirls abducted by terror group Boko Haram one year ago.

Around 270 schoolgirls from Chibok, a village in Borno state, were kidnapped by the terrorists on 14 April. Shortly after, some 50 girls managed to escape but the rest are still missing amid reports they are being raped, forced to marry their abductors and used as suicide bombers.

Who is Boko Haram?

Boko Haram fights against Western influence in Nigeria and aims to impose its version of Sharia law in the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

Boko Haram has raided several cities in the north of the country in a bid to take control of more land.

Three states, Adamawa, Borno and Yobe, have been under a state of emergency since May 2013, due to Boko Haram's deadly attacks.

Seventeen-year-old Malala, who rose to international fame after she survived an attempted murder by the Pakistani Taliban for her advocacy of free education for girls, said Nigerian leaders and the international community must do more to help the girls.

"In my opinion, Nigerian leaders and the international community have not done enough to help you," she said in a letter to the girls, whom she called "my brave sisters".

"I am among many people pressuring them to make sure you are freed," Malala continued. "Remember that one day your tragic ordeal will end, you will be reunited with your families and friends, and you will have the chance to finish the education you courageously sought.

"I look forward to the day I can hug each one of you, pray with you and celebrate your freedom with your families. Until then, stay strong, and never lose hope. You are my heroes."

Malala wrote the letter on the eve of the first anniversary of the mass kidnapping. Last July, she urged the Nigerian government to focus on the search of the missing girls during a meeting with the girls' relatives in Abuja, Borno State.

"If you don't focus on the future generation it means you are destroying your country. Think about these girls," she said during the meeting.

As the world marks the first anniversary of the girls' disappearance, Amnesty International released a report warning that Boko Haram still abducts, tortures, rapes and kills thousands in northern Nigeria.

The UN Children's Fund (Unicef) reported some 800,000 children have been displaced due to violence by the terror group, which recently pledged allegiance to Islamic State (Isis) terror group.

Nigeria: Humanitarian situation is ‘dire’ under Boko Haram threat, says UN IBTimes UK