Liverpool boss Jurgen Klopp has admitted to finding it "really difficult" watching scenes unfold as former club Borussia Dortmund were targeted in a suspected terrorist attack.

The team bus carrying the German side to Tuesday's Champions League tie with AS Monaco was caught up in three explosions, which police believe has Islamist links following the discovery of a message near the scene.

Dortmund's players were visibly shaken as they made their way off the bus, with defender Marc Bartra taken to hospital with a fractured wrist, and Klopp is "proud" of the way his ex-employers handled the situation after being forced to play the quarter-final tie less than 24 hours later.

"It was a really difficult moment for me because a lot of my friends are on the bus," he told reporters. "I was really scared and concerned for them. I had contact with a few people but I didn't want to bother them with my silly questions

"I can understand both sides [on when to play game]. I think everyone would understand if they didn't play. I was really proud of how Dortmund handled it. After game I saw faces of my former players and shock in their eyes.

"It will take time to deal with it. If someone had offered the people on the bus whether to play the game I'm sure they wouldn't have."

Dortmund went on to lose the rearranged first leg 3-2 at the Westfalenstadion on Wednesday night, with the return match scheduled to take place in six days' time.