China reported more cases of the highly transmissible Omicron coronavirus variant on Monday, with authorities on high alert over flare-ups in major cities.

Authorities are already battling several outbreaks – including in Xi’an, the capital of the northwest province of Shaanxi.

The 13 million city residents of the sprawling city are now in their third week of lockdown.

Fears grew about a cluster of infections in the northern city of Tianjin, which was linked to two Omicron cases reported on Monday in the city of Anyang around 400 kilometers away in central China’s Henan Province.

“The general public should not leave Tianjin unless essential to do so,” city officials said in a statement on Sunday.

Those who need to leave must obtain official permission and test negative for the virus within 48 hours of their departure, it added.

Schools and university campuses have been closed, and trains into Beijing from Tianjin have been canceled. Road checkpoints have been set up for vehicles entering the capital.

Tianjin – just 150 kilometers from Beijing – has already ordered the testing of all 14 million residents.

Another 21 cases were reported in the city on Monday, although the strain of virus wasn’t confirmed.

Footage from state broadcaster CCTV showed masked people in Tianjin’s Nankai district queueing for virus tests on Sunday from medical workers in white hazmat suits.

Gu Qing, director of the Tianjin Health Commission, told reporters at the weekend that the new cases are “mainly students and their parents who go to the same nursery and school.”

The world’s most populous nation slowed the spread of the virus to a trickle through its zero COVID strategy, which includes targeted lockdowns, border restrictions and quarantines.

China’s total reported cases are far lower than other virus hotspots such as the United States and Europe.

But there is widespread concern about the impact of Omicron.