The emergence of the fast-spreading variant is yet another test of China's zero Covid strategy, with authorities already battling several outbreaks -- including in Xi'an where 13 million city residents were 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 Monday in the city of Anyang around 400 kilometres (250 miles) away.
"The general public should not leave Tianjin unless essential to do so," city officials said in a statement 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 cancelled.
Road checkpoints have been set up for vehicles entering the capital.
Tianjin -- just 150 kilometres from Beijing -- has already ordered the testing of all 14 million residents.
Another 21 cases were reported in the city Monday, although the strain of virus wasn't confirmed.
The world's most populous nation -- where the coronavirus was first detected in late 2019 -- slowed the spread of the virus to a trickle through its zero Covid strategy, which includes targeted lockdowns, border restrictions and lengthy quarantines.
While China's total reported cases are far lower than other virus hotspots such as the United States and Europe, there is widespread concern about the impact of Omicron.
