Langfang, which neighbors Beijing in the capital's south, started a citywide round of nucleic acid testing on Tuesday after its Gu'an county reported a confirmed case earlier in the day, the first reported in the city since the Hebei outbreak began last week.

Testing of its nearly five million residents is expected to complete in three days, and a second round of testing is also planned in some areas under its administration, such as Sanhe city which neighbors Beijing's Tongzhou district. All tests are free according to the city's anti-virus working group.

Residents in Langfang are also requested to stay home for seven days after receiving the tests.

Xiong'an New Area in Hebei's northern part will also undergo a citywide mass testing, considering there are a huge number of migrant workers in the new economic zone under construction, Duan Yunbo, deputy head of Hebei Health Commission, said in an interview with China Central Television on Monday night.

According to Duan, the cities of Dingzhou and Xinji, which are close to Shijiazhuang, are also conducting mass testing.

Shijiazhuang and Xingtai cities have started a second round of mass testing on Tuesday. The second round will be conducted faster than the first as sampling and testing methods are improved, Duan said.

As of Tuesday, Hebei has reported 305 confirmed COVID-19 patients and 227 asymptomatic carriers under medical observation, according to the province's health commission.

After the outbreak started at the beginning of the year in Hebei, mass nucleic acid testing for residents have been one of the major measures taken in the province to screen out people with COVID-19 and prevent further transmission.