The U.S. National Security Agency (NSA) used a partnership with Denmark's foreign intelligence unit to spy on senior officials of neighbouring countries, including German Chancellor Angela Merkel, Danish state broadcaster DR said.

The findings are the result of a 2015 internal investigation in the Danish Defence Intelligence Service into NSA's role in the partnership, DR said, citing nine unnamed sources with access to the investigation.

According to the investigation, which covered the period from 2012 to 2014, the NSA used Danish information cables to spy on senior officials in Sweden, Norway, France and Germany, including former German Foreign Minister Frank-Walter Steinmeier and former German opposition leader Peer Steinbruck.

Asked for comment on the DR report, a spokesperson for the German chancellery said it only became aware of the allegations when asked about them by journalists, and declined to comment further.

Danish Defence Minister Trine Bramsen declined to comment on "speculation" about intelligence matters in the media.

"I can more generally say that this government has the same attitude as the former Prime Minister expressed in 2013 and 2014 - systematic wiretapping of close allies is unacceptable," Bramsen told Reuters in a statement.

In Washington, the NSA and the Office of the Director of National Intelligence (DNI) declined to comment. A spokesperson for the Danish Defence Intelligence Service also declined to comment.

Denmark, a close ally of the United States, hosts several key landing stations for subsea internet cables to and from Sweden, Norway, Germany, Holland and Britain.

Through targeted retrievals and the use of NSA-developed analysis software known as Xkeyscore, NSA intercepted both calls, texts and chat messages to and from telephones of officials in the neighbouring countries, sources told DR.

The internal investigation in the Danish Defence Intelligence Service was launched in 2014 following concerns about former NSA employee Edward Snowden's leaks the previous year revealing how the NSA works, according to DR.

Snowden fled the United States after leaking secret NSA files in 2013 and was given asylum in Russia.

Following DR's report, Snowden posted a cryptic Danish-language comment on Twitter saying: "If only there had been some reason to investigate many years ago. Oh why didn't anyone warn us?"

Steinbruck told German broadcaster ARD he thought it was "grotesque that friendly intelligence services are indeed intercepting and spying on top representatives" of other countries.

"Politically I consider it a scandal," he said.

Swedish Defence Minister Peter Hultqvist told Swedish SVT broadcaster that he "demanded full information". Norwegian Defence Minister Frank Bakke-Jensen told broadcaster NRK that he took the allegations seriously.

In Paris, French Minister for European Affairs Clement Beaune told France Info radio that the DR report needed to be checked and that, if confirmed, it would be a "serious" matter.

"These potential facts, they are serious, they must be checked," he said, adding there could be "some diplomatic protests".

A decision in August last year to suspend the head of the Danish Defence Intelligence Service and three other officials following criticism and accusations of serious wrongdoings from an independent board overseeing the agency centred on the 2015 investigation, according to DR.

Denmark said last year it would initiate an investigation into the case based on information from a whistleblower report. That investigation is expected to be concluded later this year.