The head of the National Security Agency in the US, General Keith Alexander, has said he does not know why his agency failed to prevent Edward Snowden from stealing and leaking highly classified documents.

Mr Snowden, an intelligence contractor, fled to Hong Kong in May after revealing extensive internet and phone surveillance by US intelligence and is now thought to be in Moscow.

Speaking on ABC News' This Week programme General Keith Alexander he said : "[Snowden] betrayed the trust and confidence we had in him. This was an individual with top secret clearance whose duty it was to administer these networks. He betrayed that confidence and stole some of our secrets,"