The US First Lady Michelle Obama has described herself and President Barack Obama as being "outraged and heartbroken" over the girls' abduction in Nigerian.

Speaking instead of her husband in the weekly presidential address, she said: "What happened in Nigeria was not an isolated incident. It's a story we see every day as girls around the world risk their lives to pursue their ambitions."

Boko Haram has admitted capturing the girls, saying they should never have been in school and should get married instead. The group has also threatened to sell the girls as "slaves".

Boko Haram has been engaged in a violent campaign against the Nigerian government since 2009.

It is thought the majority of the girls are Christians, although a number of Muslims are among those who were taken.

Chibok, from where they were abducted, is a small community where families are made up of members of both faiths.