Chelsea have urged fans travelling to Naples for Wednesday's Champions League game to "exercise extreme caution" after two supporters needed hospital treatment.
The 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 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.