China has reported 71 new Covid-19 cases, the national health authority has said, as an outbreak of the highly contagious Delta variant threatens the country's eastern city of Nanjing.

Local infections account for 31 of the new cases, down from 40 a day earlier, the National Health Commission said in a statement, reports Reuters.

All of the local cases have been reported in the eastern province of Jiangsu, where Nanjing is the capital, it said.

Based on the genetic testing results of patients, the virus strain that caused Nanjing's coronavirus outbreak is the Delta strain, a Nanjing city government official told a media conference.

Nanjing is undergoing its second round of massive nucleic acid testing of its 9.3 million residents and has started a third round of testing on key regions, the government has said.