All the 13 locally transmitted positive cases of COVID-19 reported on Tuesday in the cities of Suifenhe and Dongning in Heilongjiang province were preliminarily identified in the same transmission chain, officials said on Tuesday.

"Among all the cases, five are engaged in loading and unloading operations of imported grain, one is engaged in transporting imported grain, six are their family members, and the other one lives in the same building of a case," Jing Mei, deputy director of the Mudanjiang Center for Disease Control and Prevention, said at a news conference on Tuesday night. "Although the transmission chain is relatively clear at present, there are still risks of community transmission."

On Tuesday, three confirmed cases and nine asymptomatic carriers were detected in Suifenhe and one confirmed case was reported in Dongning.

Dongning and Suifenhe are port cities on the China-Russia border under the administration of Mudanjiang in Heilongjiang.

The patients have been sent to designated hospitals for quarantine and treatment.

As of 3 pm on Tuesday, over 520 close contacts had been traced and put under concentrated quarantined, Jing said.

Suifenhe has adopted a series of measures to contain the infection, including preventing residents' unnecessary trips out of the city, Wu Ye, Suifenhe's vice-mayor, said at the news conference.

No vehicles are allowed on the roads except for those engaged in the transport of goods or engaged in epidemic prevention and control, medical aid or emergency rescue.

All public transportation services, including trains, buses, taxis and online car hailing, have been suspended.

The county has also stopped all business activity except for those involved in supplying daily necessities.