Whistleblower Edward Snowden has revealed another secret surveillance programme that gives the US government access to millions of people’s emails, online chats and browsing histories, according to leaked documents.

XKeyscore lets National Security Agency (NSA) analysts to read e-mails, online chats and track browsing data without any prior authorisation, according to training guides for the programme leaked to the Guardian by the former CIA employee.

The documents appear to shed light on one of Snowden’s earlier statements that, from his desk, he could "wiretap anyone, from you or your accountant, to a federal judge or event the president, if I had a personal e-mail."

One training slide revealed by the newspaper illustrates the digital activity constantly being collected by the programme and the analyst’s ability to query the databases at any time, yet information requests are not reviewed by a court or any NSA personnel before being processed.

Snowden told the Guardian last month that he was authorised to use XKeyscore while working as a technology consulting firm contractor for the NSA.

US officials denied the new claims.

Snowden, who recently revealed Prism surveillance programme of NSA, is still believed to be in Russia.

Earlier this month, Wikileaks founder Julian Assange said that irrespective of the outcome regarding the situation of Snowden, his information about US surveillance programmes will continue to be released.