At least 200 new coronavirus infections, in five Chinese provinces, had been linked to an outbreak at the airport of the eastern city of Nanjing.
Chinese state media reported this on Friday, as the Delta variant continued its spread.
The first infections had been detected last week among airport workers in the metropolis of 9 million people.
It was suspected that the virus reached Nanjing via a flight from Russia.
Most of the detected cases remained confined to Nanjing, the capital of Jiangsu province.
However, individual infections have also been found in other cities among travellers returning from the region.
Two infected people were also reported in the Chinese capital Beijing.
The authorities called on people to refrain from unnecessary travel.
In addition, strict quarantine rules were imposed on returnees from Nanjing, where the entire population had been tested twice for the virus since last week.
For more than a year, life has largely returned to normal in China, where the first worldwide infections with the new virus were discovered in December 2019.
The authorities have a strict “zero COVID policy,” in which outbreaks are immediately countered with mass testing, restrictions on movement outside the home, contact tracing and quarantine.
Tough restrictions were also put in place for those entering the country, who must spend at least two weeks in a quarantine facility.
However, the highly contagious Delta variant has recently pushed the authorities to their limits several times.
In recent weeks, there had been local outbreaks of the variant in the southern Chinese provinces of Guangdong and Yunnan.