Genome sequencing has revealed that all locally transmitted confirmed cases of COVID-19 in the recent outbreak in Harbin, Heilongjiang province, involved the Delta variant, officials said on Friday.

"Further tracing is underway, with the guidance of national and provincial disease control experts," Ke Yunnan, deputy director of Harbin's health commission, said at a news conference on Friday.

Harbin reported 15 new local cases on Thursday, all of them in the city's Bayan county.

All the cases were found among people in quarantine, who were immediately transported to Harbin Sixth Hospital for medical observation and treatment.

The first round of tests has been completed for all residents in the districts of Nangang, Daoli and Songbei in urban Harbin and Bayan county.

Results for all the samples — nearly 3.16 million — have been received. Twenty-nine of those were positive.

"Tests covering residents in the other six districts and eight counties in the city will be completed late on Friday," Ke said.

As of Friday, 3,691 close contacts have been identified, and 2,743 of those people have been quarantined.

Since the first case in the latest outbreak was reported on Monday, Heilongjiang has reported 26 local cases, all of them in Harbin.