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

The documents were leaked by fugitive whistleblower Edward Snowden.

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.

"We are not going to comment publicly on every specific alleged intelligence activity, and as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations," a White House spokeswoman said.          

The revelations come after Germany demanded answers from the US over allegations Chancellor Angela Merkel's phone was bugged.

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 theGuardian.

The identities of the politicians in question were not revealed.

The revelations 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.

Earlier today, German Foreign Minister Guido Westerwelle summoned US Ambassador to Germany John B Emerson to discuss the claims the US may have monitored Ms Merkel's phone.

Yesterday Ms Merkel called US President Barack Obama to demand immediate clarification.

She told him if such surveillance had taken place it amounted to a "grave breach of trust".

Responding to questions about allegations that Ms Merkel's mobile phone was bugged, Taoiseach Enda Kenny said if it is true it is an appalling situation.

Mr Kenny added that he always operated on the basis that all his calls were listened to.