Tianjin, a major Chinese port city near the capital Beijing, has begun mass testing of its 14 million residents after a cluster of 20 children and adults tested positive for Covid-19, including at least two with the Omicron variant.
Those infected are 15 students aged between eight and 13, a member of staff member at an after-school centre, and four parents. The citywide testing is to be completed over two days.
China has stepped up its zero-tolerance Covid strategy in the run-up to the Winter Olympics, which open in Beijing on February 4.
The Chinese capital is 70 miles (115km) north-west of Tianjin and connected by a high-speed rail link that takes less than an hour.
Millions of people are being confined to their homes in Xi’an and Yuzhou, two other cities that are further away but have larger outbreaks.
The city of Zhengzhou, a provincial capital 40 miles (70km) north of Yuzhou, is also carrying out mass testing and closing schools from Monday.
The first two cases confirmed in Tianjin were a 10-year-old girl and a 29-year-old woman working at the after-school centre. Both were infected with the Omicron variant.
In subsequent testing of close contacts, 18 other people had tested positive and 767 negative as of Saturday night.