Officials in the northeast China city of Harbin say national level health officials have been sent to the city to deal with what may be a coronavirus outbreak.

The city of 9.5 million people reported three infection cases Wednesday, a day after discovering a first case of community transmission.

After the initial finding, authorities started mass testing and closed schools. The city also ordered businesses like mahjong parlors, cinemas and gyms to shut. City authorities say residents must display a negative virus test to be able to leave for only essential travel. Otherwise, people are being told to stay home.

China has been able to keep the virus from transmitting widely within its borders through a costly and strict strategy that relies on lockdowns and mass testing.