German police are urging train passengers to be wary of ticket machines, which they say could explode spontaneously due to botched manipulations by thieves.

In recent months, unidentified perpetrators have been taping shut crevices, seams and openings on ticket dispensers at Deutsche Bahn  train platforms around the state of Hesse. They then fill the machines with flammable gas and set off a small explosion to crack them open, before making off with the cash inside.

But the technique doesn't always work, according to statement by federal police and Hesse state police released on Wednesday. Failed attempts could leave ticket dispensers full of gas and in danger of spontaneous explosions, the statement said.

Anyone who notices such a ticket machine is "urgently" requested to vacate the area and inform police immediately.

Just this Tuesday, a train platform in Gross-Karben was evacuated when a taped-up ticket dispenser was discovered, said police, who also reported smelling gas. Bomb experts were called in to assist.