Inter Milan have been ordered to play their next two home games behind closed doors after Napoli defender Kalidou Koulibaly was allegedly the victim of racist abuse from Milan supporters during an Italian Serie A match on Thursday (NZ time).
Koulibaly has also been handed a two-match ban by Serie A's disciplinary tribunal after he was sent off in the 81st minute for sarcastically applauding the referee for booking him.
Napoli said on their club website the Senegalese player was subject to "racist chants" during the match at Milan's San Siro stadium, without giving further details.
Napoli coach Carlo Ancelotti said some fans made animal noises throughout the entire game.
The news comes after a supporter who was with a group of Inter Milan fans died after being knocked down by a van amid violence before the match.
Four Napoli fans were hurt and one of them was stabbed, police said.
Inter's next two home matches against Benevento in the Italian Cup and Sassuolo in the Serie A will be staged behind closed doors, with no spectators.
In addition, the section of the San Siro stadium occupied by Inter's "ultra" fans has been ordered to be kept closed for a third game in the league, against Bologna.
Milan mayor Giuseppe Sala apologised to Koulibaly over the insults, describing them as "a disgrace" on his Facebook page.
Napoli lost 1-0 after conceding a stoppage-time goal, leaving them nine points behind Serie A leaders Juventus.
"I'm disappointed by the defeat and above all to have left my brothers," Koulibaly said on Twitter.
"But I am proud of the colour of my skin. Proud to be French, Senegalese, Neapolitan: a man."
Ancelotti said Napoli asked an Italian federation delegate three times during the match for the game to be suspended but instead public announcements were read out asking fans to stop.
The coach said Napoli would walk off the pitch if there was a similar incident in the future.
Inter Milan coach Luciano Spalletti said it was the sort of behaviour that was holding back Italian football.
"We need a change of mentality [if] our objective is to bring our football back to the top in Europe," he said.
Cristiano Ronaldo, currently the most celebrated player in Serie A, was among those quick to offer supportive messages to Koulibaly on social media.
Juventus's Portuguese striker posted a picture of himself playing against Koulibaly with a message in Italian.
"Education and respect are needed in the world and in football. No to racism and to any offence and discrimination," he wrote.