US spying 'errors'
Documents leaked to the Washington Post in mid-August suggested the NSA breaks US privacy laws hundreds of times every year.
The papers revealed that US citizens were inadvertently snooped on for reasons including typing mistakes and errors in the system,
In one instance in 2008, a "large number" of calls placed from Washington DC were intercepted after an error in a computer program entered "202" - the telephone area code for Washington DC - into a data query instead of "20", the country code for Egypt.
Later in August, the Washington Post reported that US spy agencies had a "black budget" for secret operations of almost $53bn in 2013.