The extent of U.S. covert surveillance at the United Nations was further detailed Sunday, according to a report by Germany's leading weekly that claimed agents hacked into video conferencing at the organization’s New York headquarters.

In a development that could further strain relations between Washington and its allies, German magazine Der Spiegel said secret documents obtained by former intelligence contractor Edward Snowden outlined how the NSA systematically spied on other states and institutions.

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

In the summer of 2012, NSA experts succeeded in getting into the U.N. 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," wrote Der Spiegel.

'Deep Concern'

Snowden's leaks have embarrassed the U.S. 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.

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

The Guardian last week 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.

Earlier this month, U.S. President Barack Obama announced plans to limit U.S. government surveillance programs, saying the United States could and should be more transparent.

The issue has also become a hot topic in Germany before an election next month. Some reports have suggested that German intelligence agents have cooperated 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 cooperation than she has so far acknowledged.

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