At least four persons have been injured in an explosion near a Munich railway station caused by an old aircraft bomb.

German police said the device exploded on Wednesday during construction work near Donnersbergerbrücke in the city's east. A cloud of smoke could be seen rising from the site, which is located near a busy railway line.

"During tunnel work near the Donnersbergerbrücke, an aerial bomb exploded during drilling," Munich police reported on Twitter.

"The danger area has been cordoned off, there is no danger outside this area," they continued.

"Currently, more than 40 officers are on duty in the area ... and we also have support from two helicopters."

Train service to and from the neighboring station has been discontinued indefinitely.

Unexploded bombs are still often discovered in Germany, even 76 years after the war ended, and are frequently found while working on building sites.

They are often defused or disposed of in controlled explosions, a process that occasionally necessitates precautionary large-scale evacuations.