German investigators are hunting for possible suspects responsible for three explosions that rocked Dortmund football team's bus, injuring a player.

The assault, described by Dortmund city's police chief as a "targeted attack" against the team, shook German football before a Champions League quarter-final at home to Monaco.

Borussia Dortmund's team bus was attacked with explosives on Tuesday shortly before the start of the match, injuring defender Marc Bartra and forcing the quarter-final to be postponed by a day.

German police said they did not know who was behind the attack, in which three explosions went off at 7:15pm near the hotel where the team was staying, but said the team appeared to be the target.

Investigations will also focus on a letter claiming responsibility for the attack that was found close to the site of the blasts.

"The letter claims responsibility for what happened," prosecutor Sandra Luecke said late Tuesday, telling journalists that "its authenticity is being verified".

Football at its finest. Dortmund fans inviting the Monaco fans to stay at their houses so they don't miss the postponed game #BedForAwayFans pic.twitter.com/btvP0RPGzt — Ahsan (@Ahsi_7) April 12, 2017

German authorities have held off from describing it as a terror attack, saying that it is too early to determine the motive.

The blast shattered the bus windows and the vehicle was burned on the right hand side.

"The bus turned on to the main road, when there was a huge noise - a big explosion," Dortmund's Swiss goalkeeper Roman Burki told Swiss media.

"After the bang, we all crouched down in the bus. Anyone who could, threw himself on the floor. We did not know if more would come."

Burki said Marc Bartra was "hit by splinters of broken glass". Dortmund's press spokesman said the 26-year-old had broken the radius bone in his right wrist.

Monaco fans just started singing "Dortmund". Very warmly received by Dortmund fans in the stadium. pic.twitter.com/vPVVy7J7iq — Archie Rhind-Tutt (@archiert1) April 11, 2017

Dortmund said Bartra had an operation on Tuesday after "breaking the radial bone in his arm and getting bits of debris lodged in his hand".

Dortmund's president Reinhard Rauball said he believed the team would be ready for Wednesday's game.

"The players will be able to push this out of their minds and be in a position to put in their usual performances," he said.

"The worst thing would be if whoever committed this attack was now able to get to affect them through it."

'Lot to deal with'

But ex-Dortmund player Steffen Freund, who won the Champions League with Borussia in 1997, said there would be scars.

"When there has been a direct attack on the team bus, then it's not just forgotten by Wednesday," said the 47-year-old.

"Mentally and psychologically that is hard to absorb, it's a lot to deal with."

Dortmund police said security would be tightened at Wednesday's match, with a major deployment of officers to "ensure that the game is played safely".

The club said other players were safe and there was no danger inside the Signal Iduna Park stadium.

"The news that the game had to be called off was received very calmly," Al Jazeera's Dominic Kane reported from outside the stadium.

"Many of the fans of Monaco, the opposition team, were chanting Dortmund's name - in effect expressing their solidarity with the plight of the fans, the team and the player injured in this incident."

Germany has been on high alert since a series of attacks last year, including the Christmas market truck assault in Berlin in December that claimed 12 lives.