China's northeastern city of Harbin, which has a population 10 million, has gone into semi-shutdown after reporting new locally transmitted Covid-19 cases for the first time since early February.

Three of 16 new local cases reported in China for September 21 are in Harbin, the provincial capital of Heilongjiang, the National Health Commission has said. The last reported locally transmitted cases in the city were on February 4.

Harbin, known for its minus 30 degrees Celsius winters, has vowed to complete an initial round of city-wide testing by Thursday, and has told its residents to refrain from stepping outside their homes, unless necessary, before their test results are out.

The city has already told its residents to avoid leaving town unless for essential reasons, and those who do leave must produce proof of negative test results within 48 hours from departure, Reuters reports.