Germany's federal prosecutors are investigating whether a website has committed treason.

Netzpolitik.org reported on plans to expand the country's domestic surveillance of online communications earlier in the year.

The site says it has received a letter from prosecutors announcing the probe against two of its journalists as well as an unidentified source.

The stories were published in February and April.

The February article alleged Germany's domestic intelligence agency wanted additional funds to increase its online surveillance programmes.

The April follow-up was about the spy agency's efforts to set up a special unit to monitor social networking websites.

An earlier report suggested the pair of journalists had already been charged with treason, but it has since emerged that they are in fact being investigated.

In Germany the maximum punishment for treason is 15 years in prison unless a judge decides to override that with a harsher sentence.