The U.S. National Security Agency has bugged the United Nations' New York headquarters, Germany's Der Spiegel weekly said on Sunday in a report on American spying that could further strain relations between Washington and its allies.

Citing secret U.S. documents obtained by former intelligence contractor Edward Snowden, Der Spiegel said the files showed how the United States systematically spied on other states and institutions.

Der Spiegel said the European Union and the UN's Vienna-based nuclear watchdog, the International Atomic Energy Agency, were among those targeted by U.S. intelligence agents.

Sunday's news is apparently the latest in a series of disclosures of highly classified files from Snowden showing the massive extent of U.S. spying. (Ewen MacAskill/Guardian/AP)

In the summer of 2012, NSA experts succeeded in getting into the UN video conferencing system and cracking its coding system, according one of the documents cited by Der Spiegel.

"The data traffic gives us internal video teleconferences of the United Nations (yay!)," Der Spiegel quoted one document as saying, adding that within three weeks the number of decoded communications rose to 458 from 12.

Internal files also show the NSA spied on the EU legation in New York after it moved to new rooms in autumn 2012. Among the documents copied by Snowden from NSA computers are plans of the EU mission, its IT infrastructure and servers.

According to the documents, the NSA runs a bugging program in more than 80 embassies and consulates worldwide called "Special Collection Service". "The surveillance is intensive and well organized and has little or nothing to do with warding off terrorists," Der Spiegel wrote.

Global spying exposed

Snowden's leaks have embarrassed the United States by exposing the global extent of its surveillance programs.

Washington has said its spies operate within the law and that the leaks have damaged national security. But some of the Snowden revelations have shown that the NSA broke privacy rules or overstepped its legal authority thousands of times a year over the last half-decade, conducting illegal eavesdropping on U.S. citizens and people in the United States.

Bugging the UN would also be offside, violating a no-spying deal between the global body and the United States.

A week ago Britain, a staunch U.S. ally in the intelligence field and the subject of some of Snowden's leaks, detained the partner of a journalist working for London's Guardian newspaper who has led coverage of the story. British police said documents seized from David Miranda were "highly sensitive" and could put lives at risk if disclosed.

The Guardian last week also destroyed computer equipment containing Snowden files after it was threatened with possible legal action by senior British government advisers.

In an open letter to British Prime Minister David Cameron published on Sunday, editors of leading Nordic newspapers said Miranda's detention and moves against the Guardian were "undermining the position of the free press throughout the world".

We are "deeply concerned that a stout defender of democracy and free debate such as the United Kingdom uses anti-terror legislation in order to legalise what amounts to harassment of both the paper and individuals associated with it," said the letter from Sweden's Dagens Nyheter, Finland's Helsingin Sanomat, Denmark's Politiken and Norway's Aftenposten.

The issue has also become a hot topic in Germany before an election next month. Some reports have suggested that German intelligence agents have co-operated with U.S. spies.

There could be a voter backlash if it emerges that Chancellor Angela Merkel, tipped to win a third term, knew more about such co-operation than she has so far acknowledged.