SHANGHAI — China's two leading search engine operators, Baidu and Sogou, were fined on Thursday for their negligence in publishing unchecked advertising for unlicensed medical services and private companies.

The fines were issued by the Shanghai Industrial and Commercial Bureau on charges of publishing false and illegal advertisements.

Baidu was fined 28,000 yuan (about $4,000) as it linked commercial ads of private hospital groups with certain key word searches, which pointed to medical services that the hospitals are not qualified for.

The hospitals were also given fines of up to 46,000 yuan.

Sogou was fined 10,300 yuan for carrying an advertisement containing false messages and for a company whose business license had been revoked.

The regulator said the fines were issued in line with a provisional regulation on internet advertisements that took effect on September 1, 2016. The regulation holds search engines responsible for censoring online ads they publish.

Ying Jun, advertisement supervision official with the bureau, said the administrative fines can negatively affect the credit of the advertisers. The bureau will tighten supervision of internet ads, including those on personal social media accounts.

The search engine operators have long faced accusations over their online ad business. A 21-year-old student died in May 2016 after spending a fortune on an ineffective treatment for his illness, which he learned about from a Baidu-promoted link to an unqualified hospital.

Advertisers often spend billions of yuan to have their ads placed on top of search results via the search engines.