Chelsea have urged fans travelling to Naples for Wednesday's Champions League game to "exercise extreme caution" after two supporters needed hospital treatment.
The Italian city has a long history of cases of away supporters being targeted by Napoli ultras.
"The club is aware of an incident that took place on Tuesday evening in Naples," a Chelsea statement said.
"Two fans are being treated in hospital, having sustained non-life-threatening injuries. The club would like to remind all supporters to exercise extreme caution whilst in the city and take note of the advice shared ahead of this fixture."
Fans on social media claimed Chelsea supporters had been stabbed, but there has been no official confirmation of how the injuries were caused.
Around 2,500 Chelsea supporters have tickets for the game, with fans urged to meet at designated areas so they can travel to the ground by shuttle bus under police escort.
In travel advice that was issued to fans earlier this month, Chelsea said: "During Uefa competition matches, attacks on away fans have sometimes occurred in Naples."
Supporters have been urged not to walk around the city and to avoid wearing club colours or emblems that identify them as Chelsea fans.
Liam Rosenior's side need a win against Napoli at the Stadio Diego Armando Maradona to go directly into the last-16 stage of the Champions League.
Their hosts - Serie A champions last season - are seeking a win to stay in the competition.
