China is sealing off cities in a northeastern province that borders North Korea as a growing cluster of cases threatens to undermine its hard-won containment of the coronavirus epidemic.

Jilin city, the second-largest city in Jilin province, saw bus and rail services halted and residential compounds closed off on Wednesday after the discovery of six new cases of infection. Recently reopened schools were closed again.

These six people had contact with another cluster in the adjacent city of Shulan, which was earlier put under lockdown on Sunday, suggesting that some amount of undetected spread has already taken place before authorities ordered renewed restrictions.

Overall, at least 22 infections spread across three cities in two provinces — Jilin city and Shulan in Jilin province, and Shenyang city in Liaoning province — have now been linked, making it one of China’s biggest clusters that has emerged in months.

While there has been no confirmed link between the cluster and North Korea, the cities’ proximity to the isolated country has ignited fears that an outbreak there is spilling over to China. North Korea has yet to confirm any covid-19 infections, but the U.S. military said it suspects there are cases, and Kim Jong Un’s regime has accepted help from other nations to fight the virus.

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China is on high alert against any resurgence of the virus ahead of its annual political gathering now set for later this month in Beijing. Earlier this week, Wuhan said it would test its entire population of 11 million after a handful of new cases were detected for the first time since its lockdown lifted on April 8.

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The 4.5 million residents of Jilin city are now allowed to exit city limits only after testing negative for coronavirus and completing self-quarantine, said a local health authority statement on Wednesday. Buses and train services have been halted, in movement restrictions reminiscent of the Jan. 23 lockdown of Wuhan.

Surrounding cities in Jilin province have also shut down entertainment venues, and implemented strict controls on access to public places.

“There are no pedestrians on the street, only some sporadic cars,” said Wei Hong, 19, a resident in Jilin city’s hardest-hit area of Fengman district.

Jilin will implement epidemic control measures that are similar to those in “high-risk” urban areas to prevent spread of the disease, Vice Mayor Gai Dongping said at a briefing on Wednesday. The city is conducting an epidemiological investigation under the guidance of a national expert team and more than 2,500 people have been screened so far with 367 close contacts of the confirmed infections tracked down, he said.

China’s northern border with Russia — which now has one of the biggest outbreaks in Europe of more than 232,000 sickened and 2,000 dead — also poses a threat. Strict controls have been imposed in the Chinese province of Heilongjiang that borders Russia requiring 35 days of quarantine for travelers.