North China municipality of Tianjin launched city-wide nucleic acid testing on Sunday after the city reported locally transmitted Omicron cases, according to the city's centers for disease control and prevention.

The first two cases found on Saturday morning were confirmed to be the VOC/Omicron variant (BA.1 evolution branch), said the centers on Sunday.

The variant doesn't belong to the first Omicron variant reported on the Chinese mainland on Dec 13, which was an imported case from Poland, the Tianjin centers said.

The two patients have no travel history to other provinces or overseas and did not come in contact with the first imported Omicron case on the Chinese mainland.

By 9 pm on Saturday, a total of 20 locally transmitted cases were reported in Tianjin, and 15 of them were students of primary and middle schools.

The source of the outbreak is under investigation.

The city announced massive nucleic acid tests on Sunday morning.

Zhang Ying, vice-director of the centers, said the city was likely to see more cases.