German media reported on Thursday that Angela Merkel's Chancellery has been included in an ongoing investigation into a recent WikiLeaks release of confidential documents.

Broadcaster NDR reported on Thursday that according to unnamed sources, public prosecutors in Berlin are investigating employees of the Chancellery in their search to find who sent confidential documents to WikiLeaks.

But the sources stressed that the investigation does not necessarily mean that there is a concrete suspicion against any individual employee.

Government sources also told Reuters that the Chancellery had agreed weeks ago to the investigation against "unknown" persons, but added that there are no firm suspicions against Chancellery officials.

Spiegel had previously reported that the Bundestag (German parliament) was also under investigation.

The investigation surrounds thousands of documents published by WikiLeaks in December relating to a German parliamentary inquiry into the spying collaboration between the US National Security Agency (NSA) and the German BND intelligence agency.

Many of the documents were deemed “only for official use” - the lowest level of secrecy classification.