Japan has suspended imports of buffalo mozzarella from Italy, after reports that high levels of dioxin have been found in the cheese.
South Korea halted imports at the weekend amid fears that some cheese had been made with milk contaminated with the carcinogenic chemical compound.
The European Commission has asked Italy for urgent information, adding that there is no proof of contamination.
Police are investigating whether feed given to herds near Naples was tainted.
The area, which produces large quantities of mozzarella, has become inundated with rubbish piled up in the streets, as the garbage disposal system has ground to a halt.
The Naples mafia, or Camorra, is heavily involved in waste disposal in the Campania region around the city.
But the Italian Agriculture Minister Paolo De Castro has denied that there is any link between the rubbish crisis and mozzarella production.
He also denounced what he called "a negative campaign that risks having an important economic and social impact on all products from Campania".
Japan has asked the Italian authorities for more information, though an official said that the problem appeared to have affected only a small number of producers.
"If we could be provided with the names of these firms, we could again allow cheese from the other producers to enter the country", he said.
Earlier cases of contamination in 2001 and 2003 prompted the European Commission to ask Rome to carry out regular testing.
A commission spokesman said that if the results from 2007 showed any contamination, the authorities would have to take emergency measures to protect the internal market.
Health fears first surfaced in January, since when sales of buffalo mozzarella in Italy have dropped by 30-35%.
Over 20,000 people are employed in the production of the cheese, making it a significant industry in one of Italy's poorer regions.
Although it is best known for its use melted on pizza, buffalo mozzarella is often eaten on its own, or in a salad, and costs at least twice as much as mozzarella made with cows' milk.