A northeast China province is offering cash rewards of up to 5,000 yuan ($700) for help in catching people who illegally cross the Russian border after a flood of imported coronavirus cases.

Weeks after it drastically cut international flights and banned entry to foreigners to prevent a resurgence of the coronavirus crisis, China is still struggling to contain a spike in imported cases.

Most of the new infections have been in Chinese citizens returning home.

A new front has emerged in Heilongjiang province, which on Tuesday reported 79 new cases in arrivals from neighbouring Russia, taking its total of imported infections to 326.

Provincial authorities said Monday that people who report illegal border crossings will receive a 3,000-yuan reward.

"If the citizen captures and hands over (offenders) to relevant departments, there will be a one-off reward of 5,000 yuan," the virus prevention and control working group said in a statement.

This is "to step up prevention and control work against imported cases in the province", it added.

Most of China's new coronavirus infections have been found in recently returned citizens
Most of China's new coronavirus infections have been found in recently returned citizens AFP / Hector RETAMAL

Most of the province's imported cases were discovered in the remote border city of Suifenhe, whose land crossing -- the busiest between China and Russia, with over one million travellers annually -- has been closed since April 7.

The city with a population of some 70,000 has been under strict lockdown since last week, with all public gatherings banned from Sunday.

Local officials said a makeshift hospital was completed and ready for use on Tuesday, and medical experts have been dispatched from nationwide to the area.

Suifenhe and the provincial capital of Harbin now require all overseas arrivals to quarantine for 28 days and undergo tests for the virus.

Russia on Monday reported more than 2,500 new infections -- its highest daily rise yet -- and President Vladimir Putin warned officials to brace for "extraordinary" scenarios.

Chinese Foreign Ministry spokesman Zhao Lijian urged local governments in China and Russia on Tuesday to "take the necessary measures" to prevent and stop illegal border crossings.

Zhao estimated that there are currently more than 100,000 Chinese citizens in Russia.