The United States monitored the phone conversations of 35 world leaders according to classified documents leaked by fugitive whistleblower Edward Snowden, Britain's Guardian newspaper said.

Phone numbers were passed on to the US National Security Agency (NSA) by an official in another government department, according to the documents, the Guardian said on its website.

It added that staff in the White House, State Department and the Pentagon were urged to share the contact details of foreign politicians.

The revelations come after Germany demanded answers from Washington over allegations Chancellor Angela Merkel's phone was bugged, the worst spat between the two countries in a decade.

The White House did not deny the bugging, saying only it would not happen in future.

"In one recent case, a US official provided NSA with 200 phone numbers to 35 world leaders," reads an excerpt from a confidential memo dated October 2006 which was quoted by the Guardian.

The identities of the politicians in question were not revealed.

The revelations in the centre-left Guardian suggested that the bugging of world leaders could be more widespread than originally thought, with the issue set to overshadow an EU summit in Brussels.

No immediate comment on the report was available from the NSA.