China's Wuhan city will test all 12 million residents for the coronavirus.
That's after the place where it emerged in late 2019 confirmed its first domestic cases of the highly transmissible Delta variant.
Wuhan had reported no local coronavirus cases since May 2020, but on Monday (August 2) authorities confirmed three new cases of the Delta variant.
The vice director of Hubei's provincial disease control center says these were linked to infections found in Huaian city in Jiangsu province.
The outbreak in Jiangsu is believed to have begun in the provincial capital of Nanjing, with the Delta variant mostly likely introduced on a flight from Russia, according to officials.
Since then, numerous cities in southern China and a few in the north including Beijing have reported infections.
But it was not immediately clear if all those cases were of the Delta variant, or if they were all linked to Nanjing, as some authorities have not disclosed conclusive results of their virus-tracing efforts.
The Delta variant poses new risks for the world's second-biggest economy as it spreads from the coast to inland cities.
Authorities in numerous cities have launched mass testing to identify and isolate carriers.