Nobel laureate Malala Yousafzai has urged hundreds of Nigerian schoolgirls kidnapped by Islamist militants Boko Haram a year ago to "stay strong, and never lose hope".

In an open letter to the girls to mark the first anniversary of the devastating abduction raid on their school in north east Nigeria, Peace Prize-winner Malala, 17, wrote that she and millions like her around the world "keep you and your families foremost in our thoughts and prayers".

The Pakistani teenager, shot in the head by the Taliban in 2012 after speaking out for girls' rights to education, also repeated her criticism of Nigerian political leaders and the international community for not doing enough to secure their release from captivity.

In the letter, posted on her website, Malala wrote: "We cannot imagine the full extent of the horrors you have endured. But please know this: we will never forget you. We will always stand with you.

"Today and every day, we call on the Nigerian authorities and the international community to do more to bring you home. We will not rest until you have been reunited with your families."

Malala spent her 17th birthday last July meeting the families of some of those taken in the raid on the village of Chibok, which focused worldwide attention on the deteriorating security situation in northern Nigeria. Of the 276 girls abducted, some 219 are still believed to be held by Boko Haram.

In her letter, Malala wrote that there are reasons for the women to have hope and optimism, saying Nigerian forces are retaking territory. She also wrote that the country's newly-elected president, Muhammadu Buhari, pledged to make their return a priority and would not tolerate violence against women.

She added: "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."

A damning report released today by Amnesty International says more than 2,000 women and girls have been kidnapped by the militant group to become sex slaves or indoctrinated fighters since the start of 2014. Many are being forced to become soldiers and take part in brutal attacks, sometimes on their own home villages.

The report, Our Job Is To Shoot, Slaughter And Kill: Boko Haram's Reign Of Terror, also found evidence of regular conscription and systematic execution of men and boys, claiming the Muslim extremists have slaughtered around 5,500 civilians since the start of last year.

Amnesty secretary general Salil Shetty said: "The evidence presented in this shocking report, one year after the horrific abduction of the Chibok girls, underlines the scale and depravity of Boko Haram's methods.

"Men and women, boys and girls, Christians and Muslims, have been killed, abducted and brutalised by Boko Haram during a reign of terror which has affected millions. Recent military successes might spell the beginning of the end for Boko Haram, but there is a huge amount to be done to protect civilians, resolve the humanitarian crisis and begin the healing process."