Al Qaeda is planning attacks on high-speed trains in Europe and the authorities in Germany have stepped up security on the country's rail system, a German newspaper reported on Monday.

The information about the planned attacks came from the United States' National Security Agency (NSA), which apparently intercepted a call between senior al Qaeda members several weeks ago, the mass-circulation daily said.

But the German Interior Ministry said it regularly received information about such threats and was not planning to increase overall security.

"It is known that Germany, along with other Western states, is a target for jihadist terrorists so we always assess warnings on a case-by-case basis but we already have a high level of protective measures and we do not plan to step these up at the moment," spokesman Jens Teschke said at a routine government news conference.

The scandal surrounding the NSA's global electronic spying operation has become a major headache for Chancellor Angela Merkel ahead of the September 22 election. Government snooping is a sensitive subject in Germany due to the heavy surveillance of citizens in the former communist East and under Hitler's Nazis.

Bild said German authorities had tightened security on high-speed Intercity-Express (ICE) routes and at stations with "invisible measures", including the deployment of plain clothes police officers.

A spokesman for the German federal police said efforts were already commensurate with the "highly dangerous situation both at home and abroad" but said it had alerted its forces.

The newspaper report cited unnamed security experts as saying the attacks could include acts of sabotage on rail infrastructure or bombings onboard trains.

A spokeswoman for German rail operator Deutsche Bahn would not comment on the Bild report, but said the company was always in regular contact with the security authorities over possible threats.

Earlier this month the United States shut around 20 embassies and consulates in the Middle East and Africa after saying it had picked up information through surveillance and other means about unspecified threats.

Germany narrowly avoided an attack in 2006 when two suitcase bombs left on commuter trains in Cologne failed to explode.