The documents ordered declassified on Wednesday by the US intelligence director James Clapper showed that the National Security Agency (NSA) may have collected up to 56,000 emails and other communications by Americans annually between 2008 and 2011.

A footnote on one of the formerly top-secret documents from the Foreign Intelligence Surveillance Court, which has to approve activities, indicates that the gathering of "wholly domestic communications" was unintentional. However, upon learning of the email snooping, the court ordered the NSA to take steps to remedy the problem. The NSA responded by creating new technology to filter out data most likely to include US emails and limit access to it.

In notes that accompanied the documents online, Clapper said he had decided to release the information "in the interest of increased transparency."

This comes as the NSA and other US and British intelligence agencies have come under increasing public scrutiny for their activities, particularly after revelations from documents made public by Edward Snowden.

It appears to be part of a strategy aimed at restoring public trust in the work of US intelligence agencies, following an announcement by President Obama a fortnight ago that he intended to introduce new measures to help make such activities more transparent.

The US wants to try the former NSA contractor Snowden, since granted temporary asylum in Russia, on espionage charges.

Earlier this week, the Guardian newspaper - whose writer Glenn Greenwald has reported many of the revelations - announced that it had been forced to destroy hard drives that contained documents received from Snowden. Earlier, British police used the country's anti-terror laws to detain and question Greenwald's partner, David Miranda, as he was in transit at London's Heathrow Airport.

pfd/mkg (AFP, AP, dpa, Reuters)