The US National Security Agency bugged the United Nations' New York headquarters, German weekly magazine Der Spiegel has reported.

Citing secret US documents obtained by former intelligence contractor Edward Snowden, Der Spiegel said on Sunday the US systematically spied on other states and institutions.

In the summer of 2012, NSA experts cracked the UN video conferencing system's codes, according to one of the documents cited by Der Spiegel. "Within three weeks the number of decoded communications rose to 458 from 12", the magazine said.

Der Spiegel said UN's Vienna-based nuclear watchdog, the International Atomic Energy Agency, was among those targeted by the US.

Internal files also show the NSA spied on the EU legation in New York after it moved to new rooms during the autumn of 2012.

Among the documents copied by Snowden from NSA computers were plans of the EU mission, its IT infrastructure and servers.

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

Snowden's leaks have embarrassed the United States by exposing the global extent of its spying. Washington has said its spies operate within the law and that the leaks have damaged national security.

Earlier this month, US President Barack Obama announced plans to limit government surveillance programmes, saying the US could and should be more transparent.