The US military has filed murder charges against Omar Khadr, a Canadian who was a teenager when he was captured in Afghanistan and sent to the Guantanamo Bay prison in 2002.

A London-based human rights group accused the US military of failing to adequately investigate the latest allegations of prisoner abuse at Guantanamo Bay and urged officials to open the detention centre to more independent monitors.

The US Pentagon has ordered an investigation into possible abuse at the Guantanamo Bay prison after a Marine officer said she overheard guards boasting about beating detainees.