Four Real Madrid players, including Kylian Mbappé and Vinícius Júnior, are being investigated for alleged "indecent conduct" during the Champions League game at Atlético Madrid on March 12, UEFA said on Thursday.

Madrid reached the quarterfinals by beating Atlético 4-2 on penalties at the Metropolitano stadium in the round-of-16 second leg after being 2-2 on aggregate.

UEFA said it has appointed an ethics and disciplinary inspector to study the unspecified allegations in the case, which also involves Antonio Rüdiger and Dani Ceballos.

The statement added that "further information regarding this matter will be made available in due course."

Rüdiger, Vinícius and Mbappé all started against Atlético, while Ceballos did not feature in the game.

Video clips circulating on social media showed Mbappé grabbing his crotch during celebrations on the field for Madrid winning a penalty shootout.

Any ban imposed would force a player to miss Madrid's game at Arsenal on April 8 in the first leg of the quarterfinals.

Rüdiger scored the decisive spot kick when Madrid advanced, then led teammates in an exuberant dancing run around Atlético's home field.

In a similar case prosecuted by UEFA at the European Championship last year, Madrid player Jude Bellingham was given a one-game ban suspended for a probationary period of one year.

Bellingham had scored a stoppage-time goal for England to level a round-of-16 game against Slovakia when he gestured his hand toward his crotch. England went on to win 2-1 in extra time.

UEFA disciplinary judges found Bellingham guilty of "violating the basic rules of decent conduct" and fined him €30,000 ($32,400).