REPUBLIKA.CO.ID, LONDON - The National Security Agency monitored the phone conversations of 35 world leaders after being given the numbers by an official in another US government department, according to a classified document provided by whistleblower Edward Snowden. 

In an article written by The uardian, the confidential memo revealed that the NSA encouraged senior officials in its "customer" departments, such as the White House, State and the Pentagon, to share their "Rolodexes" so the agency could add the phone numbers of leading foreign politicians to their surveillance systems. The document noted that one unnamed US official handed over 200 numbers, including those of the 35 world leaders, none of whom was named. These were immediately "tasked" for monitoring by the NSA.

The revelation is set to add to mounting diplomatic tensions between the US and its allies, after the German chancellor Angela Merkel on Wednesday accused the US of tapping her mobile phone.

Merkel, who, according to Reuters, suspected the surveillance after finding her mobile phone number written on a US document, is said to have called for US surveillance to be placed on a new legal footing during a phone call to President Obama.

"The [German] federal government, as a close ally and partner of the US, expects in the future a clear contractual basis for the activity of the services and their co-operation," she told the president.

The leader of Germany's Green party, Katrin Goring-Eckhart, called the alleged spying an "unprecedented breach of trust" between the two countries.

After Merkel's allegations became public, White House press secretary Jay Carney issued a statement that said the US "is not monitoring and will not monitor" the German chancellor's communications. But that failed to quell the row, as officials in Berlin quickly pointed out that the US did not deny monitoring the phone in the past.

Arriving in Brussels for an EU summit Merkel accused the US of a breach of trust. "We need to have trust in our allies and partners, and this must now be established once again. I repeat that spying among friends is not at all acceptable against anyone, and that goes for every citizen in Germany."

The NSA memo obtained by the Guardian suggests that such surveillance was not isolated, as the agency routinely monitors the phone numbers of world leaders – and even asks for the assistance of other US officials to do so. The memo, dated October 2006 and which was issued to staff in the agency's Signals Intelligence Directorate (SID), was titled "Customers Can Help SID Obtain Targetable Phone Numbers".