China’s Wuhan city will test its 12 million residents for the coronavirus after confirming its first domestic cases of the highly transmissible Delta variant.

Wuhan, where the virus first emerged in late 2019, had reported no local coronavirus cases since mid-May last year but on Monday authorities confirmed three cases of the Delta variant. The strain has been found in a handful of provinces and big cities including Beijing over the past two weeks.

“To ensure that everyone in the city is safe, city-wide nucleic acid testing will be quickly launched for all people to fully screen out positive results and asymptomatic infections,” Wuhan official Li Qiang told a news briefing.

Parts of an industrial and technology zone were sealed off, a measure rarely seen in the city since a lockdown last year.

The new cases in Wuhan, along with infections in the nearby cities of Jingzhou and Huanggang since Saturday, were linked to cases in the city of Huaian in Jiangsu province, said Li Yang, vice-director of the Hubei province disease control centre.

The outbreak in Jiangsu is believed to have begun in the provincial capital of Nanjing in late July, with the Delta variant mostly likely introduced on a flight from Russia, officials have said.

A Nanjing official said on Monday that even after the first cases were reported there, some shops did not rigorously check customers’ digital health credentials and some did not wear masks properly.