Communities in Shanghai that are no longer in lockdown should not slack off, a Shanghai health official warned on Tuesday, a day after the city introduced measures allowing low-risk areas to reopen.

The Shanghai government has used a three-tiered division system to carve the city up, enabling the loosening of lockdown rules in places where COVID-19 cases are nonexistent for two weeks.

People living in more than 40 percent of Shanghai's residential compounds were cleared to venture out of their homes after weeks of staying in since the city became the worst hit of China's latest COVID-19 surge.

Shanghai reported 22,348 new asymptomatic infections on Monday, down from 25,173 the previous day, while symptomatic cases edged up to 994 from 914.

People should keep observing the principle of "not going out unless necessary," and complete two antigen tests per week, the official said.

If positive cases are detected, these communities will be reclassified as zones that need to be locked down again, he said.

Some 10,000 of the 17,649 residential units in Shanghai are still in lockdown. Authorities said a list of COVID-free drug stores, convenience stores and supermarkets will be reopened to ensure daily supplies.

In areas where lockdowns were lifted, non-essential businesses will remain closed, while chain restaurants can gradually reopen with takeout services, the authorities said. 

Some 6,000 people were also discharged from hospitals on Monday, taking the total number of those being released from medical observation to 20,000 since March 1.