China's coronavirus cases have hit a seven-month high, after a cluster at a testing site helped drive up case numbers as the Delta variant challenges the nation's grip on the pandemic.

On Tuesday Chinese health authorities reported 143 new coronavirus infections -- 108 of them locally transmitted and dozens of cases  in recent days have been linked to a COVID-19 testing site in eastern Yangzhou city.

And now in a sign of anxiety over even relatively minor outbreaks, several officials have been issued warnings for mishandling mass testing, which city authorities said allowed the virus to spread.

The city of about 4.6 million people has so far conducted five rounds of widespread testing, collecting 1.6 million samples in an attempt to stamp out the spread.

State media has described the current outbreak — which has sparked local lockdowns, mass testing and travel restrictions — as the most severe since the virus initially emerged in the central city of Wuhan in December 2019.

The latest surge started after infections among airport cleaners in neighbouring Nanjing city sparked a chain of cases across the country.

Tuesday's numbers are the highest since January, when the country logged 144 new cases and 126 domestic infections, mostly in the northern regions.

Authorities are now working to shore up confidence that the latest resurgence is controllable.

"We have successfully contained the epidemic in Guangzhou, and the epidemic in Nanjing is gradually being put under control," China's Xinhua news agency cited infectious diseases specialist Zhang Wenhong as saying.

Delta variant evading Chinese vaccines

While China had largely reported few cases there are fears amongst experts there that current Chinese vaccines are less effective against the new strains of the coronavirus, but say they still offer some protection.

Only Chinese vaccines are currently being given in China, and authorities say more than 1.6 billion doses have been administered.

China had largely curbed COVID-19 at home after the initial outbreak that hit Wuhan before spreading around the world.

Since then, authorities have clamped down and controlled the disease whenever it pops up with quick lockdowns and mass testing to isolate infected people.

Current outbreaks across China, while still in the hundreds of cases in total, have spread much more widely than previous ones.

Many of the cases have been identified as the highly contagious Delta variant.

AFP