BERLIN (AP) - German police are warning rail travellers to be wary when using Deutsche Bahn's automatic ticket machines because criminals attempting to break into them may have left them filled with explosive gas.

Hesse state police spokesman Udo Buehler said since April, criminals have successfully blown open 10 ticket machines by taping closed all holes, filling them with gas and igniting it. They then steal the money and blank train tickets.

But in six cases, the attempts have failed, leaving the explosive gas inside, most recently on Tuesday in a train station in the town of Karben, near Frankfurt.

Mr Buehler warned the gas could potentially ignite when an unsuspecting customer uses the machine, though so far it has not.

He said incidents have also been reported in neighbouring states Thuringia and Lower Saxony.