Gene sequencing conducted on 16 samples of local COVID-19 cases reported in Yantai, a coastal city in Shandong province, showed the BA.2.3 Omicron subvariant which had not been reported before on the Chinese mainland, an official said on Monday.

"Gene sequencing analysis traces all cases to one transmission chain," Vice-Mayor Shen Jian said at a news conference on Monday morning.

"The new outbreak has features of spatial clustering, and the virus might have been spreading for days," Shen said.

Yantai reported 36 new locally transmitted COVID-19 cases on Sunday, including two confirmed patients and 34 asymptomatic carriers.

The city has taken a series of epidemic prevention and control measures to contain the spread. Over 4,870 close contacts have been traced and are now under quarantine, Shen said.