The German magazine Der Spiegel says the U.S. National Security Agency secretly monitored the UN's internal video conferencing system by decrypting it last year, according to Belfast Telegraph.

The weekly publication said that documents it obtained from American leaker Edward Snowden show the NSA decoded the system at the UN's headquarters in New York last summer.

Quoting leaked NSA documents, the article said the decryption "dramatically increased the data from video phone conferences and the ability to decode the data traffic".

In three weeks, Der Spiegel said, the NSA increased the number of decrypted communications at the UN from 12 to 458.

Snowden's leaks have exposed details of the United States' global surveillance apparatus, sparking an international debate over the limits of American spying.