The National Security Agency has been allowed to extend its dragnet of the telephone records of millions of US customers of Verizon through a court order issued by the secret court that oversees surveillance.

In an unprecedented move prompted by the Guardian's disclosure in June of the NSA's indiscriminate collection of Verizon metadata, the Office of the Director of National Intelligence (ODNI) has publicly revealed that the scheme has been extended yet again.

The statement does not mention Verizon by name, nor make clear how long the extension lasts for, but it is likely to span a further three months in line with previous routine orders from the secret Foreign Intelligence Surveillance Court (Fisa).

The announcement flowed, the statement said, from the decision to declassify aspects of the metadata grab "in order to provide the public with a more thorough and balanced understanding of the program".

According to Democratic senator Dianne Feinstein, the Verizon phone surveillance has been in place – updated every three months – for at least six years, and it is understood to have been applied to other telecoms giants as well.

The decision to go public with the latest Fisa court order is an indication of how the Obama administration has opened up the previously hidden world of mass communications surveillance, however slightly, since former NSA contractor Edward Snowden exposed the scheme to the Guardian.

The ODNI statement said "the administration is undertaking a careful and thorough review of whether and to what extent additional information or documents pertaining to this program may be declassified, consistent with the protection of national security."

The Verizon metadata was the first of the major disclosures originating with Snowden, who remains in legal limbo in the international airport in Moscow.