BERLIN (Reuters) - Nearly two dozen people were injured, two of them seriously, when two underground trains collided in the western German city of Duisburg on Tuesday, local police said.

A police official told Reuters the two trains had run into each other in a tunnel, but that the cause remained under investigation. He said 20 people had sustained minor injuries.

A transit official said the accident occurred around 1520 CET (1320 GMT) under the Ruhr river.

Local newspapers and television stations said more than 30 people had been injured in the accident.

Buses were dispatched to ferry passengers and to help keep rush-hour traffic running, the transit agency spokesman said.

Germany remains on high alert for possible militant attacks after 12 people were killed in a truck attack on a Berlin Christmas market in December 2016.