Manchester United ace Cristiano Ronaldo admits he rates Bayern Munich as a dark horse in this season's Champions League.

Many are tipping Barcelona and the English contingent of Chelsea, Liverpool and Arsenal to be the main threats to United's European crown.

But Ronaldo has admitted that he thinks Bayern would represent a big challenge for anyone after their demolition of his former club Sporting Lisbon in the last round.

"Bayern Munich are very strong," Ronaldo told United's official website.

"I watched them play against my old team, Sporting Lisbon, and I was very sad because I didn't expect Sporting to lose so heavily [12-1 on aggregate].

"But this can happen in football. Bayern are very good - they have mature players, strong players, quick players. Franck Ribery and Miroslav Klose are very impressive.

"Bayern play like a team and they look very difficult to beat."