An employee of Germany's intelligence service has been arrested on suspicion of spying for the US, reports say.

The man is said to have been trying to gather details about a German parliamentary committee that is investigating claims of US espionage.

German authorities have asked the US ambassador for "swift clarification".

The US National Security Agency (NSA) was last year accused of bugging the phone of Chancellor Angela Merkel as part of a huge surveillance programme.

The scale of the agency's global spy programme was revealed in documents leaked last year by a former intelligence contractor, Edward Snowden.

The revelations about the NSA put a strain on ties between Germany and the US and raised feeling in Germany against American surveillance.

The new allegation of American spying on an ally may make it harder for the US to get German help in its efforts to oppose Russian activity in Ukraine, and also to control Iranian nuclear ambitions.