China has put one northern city in lockdown and launched citywide coronavirus testing in another, after two local infections involving workers at a port and trade zone respectively, authorities have said.

There was one new case each in Dongning and Suifenhe, which both sit on the country's border with Russia.

In Dongning, a 40-year-old maintenance worker at a port tested positive on Thursday, triggering the latest measures.

Officials there said Saturday it would enter "wartime mode" -- temporarily suspending public bus services and road transport, while requiring anyone leaving the city to produce a negative Covid-19 test from the preceding 24 hours.

Schools will suspend classes and dine-in will be banned at restaurants, while in higher-risk areas only one member of each household will be allowed to leave the home once every two days to buy daily necessities -- for no longer than two hours each time.

In the Suifenhe case, a 39-year-old man who works as a loading and unloading worker at a trade zone tested positive.

Both cities on Friday announced the launch of citywide testing, expected to be completed in three days.

China -- where the virus first surfaced late last year -- has largely brought domestic transmission under control but recently reported local outbreaks in several cities.

State media blamed the recent clusters on imports of frozen food and other shipments.

After a recent outbreak in Chengdu, officials said the virus was found on food stored in an elderly couple's fridge.

There is currently no evidence that people can catch Covid-19 from food or food packaging, according to the World Health Organization.