Eleven coronavirus cases in the Shenzhen outbreak belong to mutated strains that appeared in the United Kingdom, according to genetic sequencing, officials said at a news conference on Sunday.

Shenzhen reported two new asymptomatic coronavirus cases on Sunday, bringing the total to 13 since the latest outbreak was detected in the city on May 21.

The two updated infections were discovered in the second round of wide public testing in the Yantian district. Shenzhen investigators looked at close contacts, secondary contacts, key groups and general contacts after the first case was spotted during a routine inspection of a crew working with international freighters in Yantian port.

The new cases are probably relevant to the previous cases at the port, eliminating the possibility of their association with outbreaks in Liaoning province, Anhui province and the city of Guangzhou, Guangdong province, said Chang Juping, deputy director of the municipal health commission.

"The epidemic outbreak in the city is currently limited to Yantian port and adjacent areas. The contagion is generally controllable, and the risk of large-scale spread is low," Chang said.

To strictly limit the spread of the virus, Shenzhen has fully implemented a closed-loop management system for ports in which crew members working with freighters must live in assigned accommodations and comply with necessary isolation measures.