Four people were injured, one of them seriously, when a World War II bomb exploded at a building site near Munich's main train station on Wednesday, emergency services said.

Construction workers had been drilling into the ground when the bomb exploded, a spokesman for the fire department said in a statement.

The blast was heard several kilometres away and scattered debris hundreds of metres, according to local media reports. 

Images showed a plume of smoke rising directly next to the train tracks.

Rail operator Deutsche Bahn suspended its services on the affected lines.

According to the fire service, the explosion happened near a bridge that must be passed by all trains travelling to or from the station. 

Some 75 years after the war, Germany remains littered with unexploded ordnance, often uncovered during construction work.

However, most bombs are defused by experts before they explode.

Last year, seven World War II bombs were found on the future location of Tesla's first European factory, just outside Berlin.

Sizeable bombs were also defused in Cologne and Dortmund last year. 

In 2017, the discovery of a 1.4-tonne bomb in Frankfurt prompted the evacuation of 65,000 people -- the largest such operation since the end of the war in Europe in 1945.