(Bloomberg) -- China warned that Covid-19 infections risk spreading through the mainland after at least 16 cases were found in cities bordering North Korea, which is struggling to contain one of its worst health crises in years.

Authorities found seven infections in northeastern Jilin province, as well as nine cases in neighboring Liaoning, on Thursday. National Health Commission officials on Friday said the source of the infections in Jilin, which has a long and often porous border with North Korea, is unclear and there’s a risk that the virus could spread further. Measures to prevent imported cases will be strengthened they said, without giving details. 

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While the case numbers are low compared with flareups in other parts of China, officials want to stamp out transmission before it turns into a bigger outbreak. Jilin province has only just emerged from a lockdown that kept most of its 24 million residents stuck in their homes for more than a month and severely hurt car production for major global automakers.

The warning also highlights the challenges of China’s Covid Zero strategy, which has seen the country isolate itself from the rest of the world. The playbook of harsh lockdowns and mass testing is faltering under the strain of the highly transmissible omicron variant and left the country stuck in a cycle of costly lockdowns as it tries to extinguish its spread.

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North Korea has reported more than 3 million “fever” cases since late April as well as 69 deaths. It has avoided using the word “Covid” to describe the rising wave of infections, likely because the state doesn’t have enough testing kits to confirm the cases were caused by the coronavirus. The country so far hasn’t sought international aid.

In China, parts of Dandong city in Liaoning, an important port for trade with North Korea, as well as three cities in Jilin, have been conducting mass testing to root out infections, according to local media.

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