The United Nations will approach the US government over a report by a German magazine that US intelligence spied on video conferences by top UN officials, a spokesman said Monday.

"We are aware of the reports, and we intend to be in touch with the relevant authorities on this," a UN spokesman, Farhan Haq, told reporters, adding that this meant the US administration.

Haq told reporters the 1961 Vienna Convention on diplomatic relations has become "well established international law, therefore member states are expected to act accordingly to protect the inviolability of diplomatic missions."

A report by Der Spiegel magazine said the US National Security Agency (NSA) had broken the encryption code to allow US intelligence to listen in to UN video conferences.

The measure also involved the UN's International Atomic Energy Agency, the report said quoting NSA documents. The IAEA has played a key role monitoring Iran's suspect nuclear program.

It was the latest in a series of revelations about US spying on embassies and UN agencies made since former US analyst Edward Snowden started revealing details of US intelligence tactics.

"Whenever we have received this information in the past we have taken it up with the relevant authorities," Haq said.

Asked about the issue, the US State Department said "the US government will respond through diplomatic channels to our partners and allies around the world when they raise concerns."

Spokeswoman Marie Harf insisted the US values and cooperates with the UN, often to "share information, including intelligence."

Der Spiegel said the NSA broke the encryption in mid-2012 and within weeks, had dramatically increased its surveillance of UN communications.

The NSA once allegedly caught the Chinese secret services eavesdropping on the UN in 2011, it added, quoting an internal report.

Der Spiegel also claims that the US agency kept tabs on the European Union after it moved into new offices in New York in September 2012.

Among documents provided by Snowden were plans of the EU's premises, which the NSA codenamed "Apalachee."

Earlier reports in Der Spiegel and Britain's Guardian newspaper have detailed alleged widespread NSA surveillance of EU offices, including diplomatic missions in Washington and at the United Nations in New York.

Revelations about NSA snooping made by Snowden have sparked outrage in Europe. (AFP)