Health authorities in China's eastern city of Qingdao have found coronavirus contamination on some packages stored by a seafood importer after two of its handlers tested positive for the virus but displayed no symptoms.

China has stepped up checks on frozen food imports and banned those from some foreign meat processing plants amid the global pandemic.

Thursday's results came during a routine test of the company's staff, although none of the two affected men's 147 close contacts tested positive, the Qingdao Municipal Health Commission said in a statement.

The importer's products and facilities generated 51 positive test results, but no tainted products made it to market, the commission said, without identifying the items or their origins.

Chinese customs have threatened a week's suspension of imports from companies whose frozen food products test positive for the virus, with third-time offenders subject to four weeks.

Heavy traces of the virus were found in the meat and seafood sections of a market in the capital Beijing that was the site of an outbreak in June.