After a deluge of imported coronavirus cases, a province in northeastern China is offering cash rewards of up to 5,000 yuan (700 USD) for help in catching people who illegally cross the Russian border.

Weeks after the cancelling international flights and banning entry to foreigners, China is still struggling to contain a spike in imported cases.

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

A new front has emerged in Heilongjiang province, which on Tuesday reported 79 new cases in the form of arrivals from neighbouring Russia. This influx brought the total of imported infections to 326.

Provincial authorities said on 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.

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, which holds a population of some 70,000, has been under strict lockdown since last week.

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

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

On Tuesday, Chinese Foreign Ministry spokesman Zhao Lijian urged local governments in China and Russia 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.