Three new domestically transmitted confirmed cases were identified in Dongning and Suifenhe, two port cities in Heilongjiang province, and in Manzhouli, another border city in the Inner Mongolia autonomous region, on Thursday, according to their health authorities.

Meanwhile, two confirmed cases and two asymptomatic carriers were identified among the close contacts of previous cases in Chengdu, Sichuan province, the Chengdu government said.

Among the first three, one identified in Heilongjiang is a constructor working at the customs office in Dongning, an important land port on the China-Russia border.

The discovery led the city to launch a Level III public health emergency response on Thursday. The city has undertaken a series of measures, including rolling out a large-scale testing effort for all residents in the urban area and two villages, Sanchakou and Wudadui, and barring residents from leaving the city without negative nucleic acid test results within 24 hours of their departure.

The case reported in Suifenhe is a man who worked at a warehouse that deals with import and export in the city.

Local authorities have upgraded the risk level of residential communities where the two live to medium.

In Manzhouli, a close contact of previous cases was confirmed as a patient on Wednesday. The city currently has 27 confirmed cases and two asymptomatic cases.

In Chengdu, the new cases on Thursday took the tally of the recent outbreak to eight confirmed and three asymptomatic cases. All four cases on Thursday were identified through the second round of nucleic tests for the close contacts of previous cases. The four had been in quarantine since Tuesday.

After the outbreak, Chengdu urgently carried out nucleic acid tests in key areas. As of Thursday, 1.1 million people have taken the test.

The source of the recent infections of COVID-19 in Chengdu is being sought, top health experts said.

Gene sequencing and a comparison of virus strains are being carried out to explore the source, said Zhu Xiaoping, an expert of epidemiology who is a part of the leading group coping with COVID-19 under the Sichuan Provincial Health Commission.

A 69-year-old woman surnamed Lu and her 71-year-old husband Zhao, who live in Taiping, a village in the city's suburban Pidu district, were identified as confirmed COVID-19 patients on Sunday and Monday, the first ones of the recent outbreak.

The virus was found on a door handle, light switch and food in the refrigerator in the couple's home, which indicated that it had been seriously contaminated, Zhu said.

Xie Qiang, director of the Chengdu city health commission, said the couple's movements were mainly limited to their home and community, including a nearby market and clinics. Thus, their close contacts were primarily family members, neighbors and doctors in the clinics.

As many people shop at the Taiping vegetable market which Lu frequented, it was very difficult to trace the source of the virus within a short period of time, according to Luan Rongsheng, a professor of epidemiology at the West China School of Public Health of Sichuan University.

The top priority is to grasp the critical period of prevention and control and strictly control the spread of the virus, Luan said. Ordinary people do not need to panic, he said. If they walk by a COVID-19 case on the street, they are not likely to become infected.