Hanover, Germany - Twenty people died and 13 were injured when a bus caught fire on an autobahn near the north German city of Hanover on Tuesday evening, police said.

The bus was taking pensioners home from a day's outing when it burst into flames at 8:45 pm (1945 GMT).

The driver pulled over to the side of the motorway and more than a dozen passengers managed to escape the inferno, but others were trapped inside and died, police said.

A police spokeswoman said no other vehicle was involved.

It was unclear what caused the blaze, but witnesses reported seeing flames dart from the toilet shortly after it had been vacated by a passenger. It was possible he had been smoking, they said.

Police said a technical defect appeared unlikely. "The tyres and the engine are intact," police spokesman Stefan Wittke said.

The interior of the bus was completely gutted by the flames. The fire brigade recovered Zimmer frames from the wreckage, indicating that some of the passengers were elderly people.

Police said that most of the bodies were burned beyond recognition.

The bus caught fire on the A2 autobahn, which runs from Germany's central Ruhr region to the capital, Berlin. The route is one of the busiest in Germany and one of the most accident-prone.

German Transport Minister Wolfgang Tiefensee expressed shock and called for a detailed investigation into the cause of the accident, one of the worst ever on German roads.

"If it turns out to be true that only a few of the passengers were able to escape the flames and get out of the bus then an examination is needed to determine whether safety standards were adhered to and whether they need to be improved." (dpa)