The U.S. National Security Agency (NSA) broke privacy rules and overstepped its legal authority thousands of times in the past two years, according to documents leaked by Edward Snowden, BBC News said.

The incidents resulted in the unauthorised electronic surveillance of U.S. citizens, according to documents published by the Washington Post.

Snowden, a former NSA contractor, has leaked top secret documents to the U.S. and British media. He has been given asylum in Russia.

On Thursday, August 15 the Washington Post posted on its website a selection of documents it said had been provided by Snowden, who fled the U.S. in June after providing documents detailing NSA surveillance programmes to the Guardian and Washington Post newspapers.

The documents purport to show that the unauthorised interception of telephone calls and emails of Americans and foreign nationals on U.S. soil resulted from errors and departures from standard agency processes, including through a data collection method that a secret U.S. surveillance court later ruled unconstitutional.

The documents offer more detail into the agency practices than is typically shared with members of Congress, the U.S. justice department, and the Office of the Director of National Intelligence.