"Member states are expected to . . . protect the inviolability of diplomatic missions," Ban told reporters in Geneva in response to a question about the latest allegations attributed to former US intelligence contractor Edward Snowden.

While refusing to comment directly on the reports that the National Security Agency (NSA) had kept tabs on the European Union's diplomatic mission in Washington, Ban stressed that "in principle, diplomatic missions should be protected, including (their) information."

The new accusations, which threaten to seriously harm relations between the United States and its European allies, surfaced Sunday in a report by German weekly Der Spiegel, citing confidential documents leaked by Snowden.

Microphones were allegedly installed in the EU's mission in Washington and the computer network was infiltrated, giving the agency access to emails and internal documents.

Der Spiegel said the EU delegation at the United Nations was subject to similar surveillance and the spying had also extended to the 27-member bloc's Brussels headquarters.

The weekly said the leaked documents showed the US secret service had targeted Germany more than any other EU country, but Monday's Guardian newspaper reported that France, Italy and Greece were also among 38 US surveillance "targets".

"The inviolability of diplomatic missions, including the United Nations and other international organisations . . . (has) been well-established by international law," Ban pointed out.