Michelle Obama has added her voice to calls for the safe return of nearly 300 kidnapped Nigerian schoolgirls. The US first lady posted a photo of herself in the White House holding a sign that says "#BringBackOurGirls". The tweet was signed "-mo," indicating that she sent it herself.

Mrs Obama is the latest high profile figure to join the campaign to rescue the girls who were abducted by Boko Haram, a Nigerian Islamist terror group.