India's 81-year-old prime minister does not own a mobile phone or use personal email, giving New Delhi "no cause for concern" about new US hacking revelations, his office has said.

It was revealed today that the US National Security Agency monitored the telephone conversations of 35 world leaders after obtaining their numbers from an official in another government department, according to a leaked document from the former US intelligence operative Edward Snowden.

The Guardian reports that a confidential memo shows the NSA encouraged senior officials in departments such as the White House, the State Department and the Pentagon to provide access to their "Rolodexes" containing the phone numbers of leading foreign politicians.

TOP STORIES OF THE DAY

Asked if Indian Prime Minister Manmohan Singh was concerned, his spokesman replied: "The prime minister doesn't use a mobile phone and he doesn't have an email account.

"His office uses email, but he has no personal email... We have no information and no cause for concern," he added.