Dozens of Chibok schoolchildren have been released by Boko Haram militants after spending more than three years in captivity.

The girls, who were abducted from a secondary school in the north east town of Chibok in 2014, were released through tense negotiations with the government, one official said, asking not to be named.

According to Reuters, over 50 children - among a group of about 220 who were initially kidnapped - were released on Saturday.

Their kidnap shocked the world and sparking a huge campaign with the slogan "Bring Back Our Girls", supported by then-US First Lady Michelle Obama and a list of celebrities.

More than 20 were released in October in a deal brokered by Switzerland and the International Red Cross, while a handful of others have escaped or been rescued.

About 100 are still missing.