Baodi district, located in the north of the coastal municipality, announced that since Sunday, up to 11,700 people are under observation, an increase of 2,500 from Friday.

The district, a hub among Beijing, Tianjin and Tangshan, Hebei province, has been the most severe hit by the novel coronavirus pneumonia in the region, with 31 confirmed cases out of the city's total 90 confirmed cases as of mid-noon, Sunday.

The district announced a batch of latest strengthened measures on Sunday, to upgrade previous measures including the complete closure of local communities, villages and towns, it said on Thursday.

The move indicates that each family could only arrange one member to go out for grocery, for every two days, and the district's transportation are under comprehensive control to tighten the entrance and exit from Sunday.

"Because of its geological location, the epidemic will impact the Beijing, Tianjin and Hebei province, so we have upgraded the measures, in a bid to prevent the Baodi Department Store to become a copy version of the Wuhan Seafood Market, the epidemic center," said Mao Jinsong, director of Baodi district.

A total of 11 entrances to Tianjin has been under control, and all the buses linking nearby provinces and cities have been halted, said Liu Daogang, vice-director, Tianjin Transportation Commission.

In addition, "we are looking for all the people who have contacts with the salespeople, their relatives and consumers who have come to the department store, so the people to be under observation will increase," he said.

A saleswoman in the Baodi Department Store, who had no contact with people from Wuhan, the epidemic center, was confirmed as the first one to infect her colleagues and people who went to the store since Jan 20.

The saleswoman went to a nearby city to sell shoes and soon got sick, said Zhang Ying, director of the infectious disease prevention office of the city's disease control and prevention center.

The confirmed cases are all her colleagues, customers to the store, and their relatives; and 194 salespeople are under centralized quarantined.

Zhang's analysis on TV, on how the Baodi has such great number of the cases and who was the first one infected, has become a hot search on social media in China since Feb 2.

In Tianjin, the authorities announced the comprehensive closure of local residential communities late on Thursday.

Gu Qing, head of Tianjin's disease control and prevention authority, said: "The epidemic is turning complicated, and many cases need several rounds of virus nucleic detections to be confirmed."

Baodi has a population of 926,700 with six streets and 18 towns, with a majority of suburban population of up to 720,000."So the preventions have become much more difficult," the district's director Mao noted.