Germany on Wednesday launched an investigation for "sabotage" after several arson attempts targeting Berlin's railways, which were blamed on left-wing extremists.

"The federal prosecutor's office today took charge of the investigation ... because of accusations related to sabotage aimed at undermining the constitution," spokesman Stefan Schmidt told AFP.

At least nine separate attacks, involving a dozen or so incendiary devices, have been recorded since Monday on railway lines in and around Berlin, interior ministry spokesman Jens Teschke told a regular news briefing on Wednesday.

Only in one case did a fire actually break out, slightly damaging signal cables, but checks on the lines have caused severe traffic disruptions.

A far-left group claimed responsibility for Monday's blaze just outside Berlin on the main line linking the capital to Hamburg, saying in an online message it had acted in protest against the German military's deployment in Afghanistan, now entering its 10th year.

Police said two more incendiary devices were found Wednesday on railway lines in Berlin, adding that these might have been there for several days.

Schmidt declined to say if the federal prosecutor's investigation had left-wing organisations in its sights, but Teschke said authorities were "on standby, ready to counter any group that might get involved in left-wing terrorism".

The state of Lower Saxony's interior minister, Uwe Scheunemann, warned for his part that "left-wing extremism was growing into left-wing terrorism".

"It's a short way to go from criminal arson and targeted murder," said Scheunemann in an interview with the Neue Osnabrücker Zeitung, pointing to left-wing attacks in Germany in the 1970s by the Red Army Faction (RAF).

The RAF took up arms against what they considered an oppressive capitalist state still riddled with former Nazis, killing 34 people in attacks on West Germany's elite and US military bases before disbanding in 1998.