German fighter jets, fitted with surveillance cameras, are guarding the country's railway lines after blackmailers threatened to sabotage train services.

Two acts of sabotage were carried out against railway lines over the weekend, but the police do not know whether they are connected to the blackmail demand.

The national rail company, Deutsche Bahn, says that blackmailers have sent four letters to the national rail company demanding $6m. The letters say that unless the money is paid railway lines will be sabotaged.

There is no clue as to who the extortionists are, but all the notes have been signed by a group calling themselves Friends of the Railway.

Tornados on standby

The German Government is understood to be very worried about the threats and the armed forces have been drafted in as part of increased security measures.

Tornado aircraft, fitted with surveillance cameras, are patrolling railway lines. Helicopters are also being used to keep guard.

In two recent acts of vandalism on the railways, screws were loosened on railway tracks.

A train narrowly escaped derailment last weekend in Berlin when it went over concrete blocks placed on the line.