Media reports say Borussia Dortmund's team bus was hit by three explosions while on its way to the Champions League semi-final against Monaco on Tuesday evening, leaving one player injured in hospital.

Citing German security sources, The Telegraph said it was a targeted attack on the team bus, shattering one of the vehicle's windows.  Police said “serious explosive devices” were used.

A letter found near the site of the attack “takes responsibility for the act,” a German prosecutor said.

According to the BBC, German investigators say they are intensively examining a letter claiming responsibility for three explosions which struck the bus.  

There were reports the devices, which were concealed in a hedge, could have detonated by a mobile phone or a garage door-opening device.

High levels of security were in place across the city on Tuesday night and there will be a large security force presence at Wednesday night's game, police said, adding they did not want to jump to conclusions that the attack was terror-related.

Spanish international, Marc Bartra, was undergoing an operation for a broken hand, a club spokesman said.

The game was postponed to Wednesday evening, with Borussia Dortmund fans offering Monaco supporters accommodation in the city so they can watch the match.  Monaco fans had chanted in solidarity with their Dortmund rivals.