In a BBC interview, Samantha Power spoke of her frustration at trying to get assistance to the millions of victims of the war in Sudan as ceasefires are repeatedly broken.

Ms Power called on the civilian authorities there to remove bureaucratic hurdles that are slowing down aid delivery and costing lives.

She also said it was heart-breaking that because of the ethnic violence in Darfur some people were fleeing to Chad for the fourth time in their lives.

The war, which broke out in mid-April between rival military factions, has forced more than two and a half million people to flee their homes.