Shanghai authorities on Thursday appealed to residents to keep cooperating with tight curbs imposed to stop COVID-19 spreading, saying they recognised their frustrations as China's most populous city entered the fourth day of a two-stage lockdown.

The plea for patience came as the Chinese financial hub, home to 26 million people, said its overall daily COVID caseload eased, for the first time in about two weeks, in an outbreak of the highly contagious Omicron variant that began about a month ago.

It reported 5,298 locally transmitted new asymptomatic cases and 355 symptomatic cases for Wednesday, compared with 5,656 local asymptomatic cases and 326 new cases with symptoms reported a day earlier. Shanghai accounted for almost 80% of local asymptomatic cases reported across the whole of China for Wednesday, and about 20% of those with symptoms.

In a letter to the city's residents, the Shanghai government saying it was grateful for citizens' efforts and acknowledged their sacrifices, as it sticks with China's "dynamic clearance" approach - detecting the virus, tracing contacts and centrally quarantining all positive cases.

"Pandemic prevention and control work have brought much inconvenience to people," it said in the letter, posted to its official WeChat social media account on Thursday.

"Some of you have been in quarantine and lockdown for a long time. The city deeply appreciates everyone's understanding and cooperation."

Shanghai is being locked down by splitting the city into two roughly along the Huangpu River, dividing the historic centre west of the river from the eastern financial and industrial district of Pudong. Authorities say this allows for staggered mass testing.

Movement curbs on residents in the eastern districts started on Monday and are set to be lifted at 05:00 local time (21:00 GMT) on Friday.

Ma Chunlei, deputy secretary general of the Shanghai government, told a daily news conference on Thursday that officials would look at testing results and confer with experts to determine how the lifting of the first stage of the lockdown would be carried out.

Areas west of the river will be locked down from 03:00 on the same day, though some housing compounds in western districts already directed residents to stay home from Wednesday.

Many across the city have taken to social media to vent their frustrations in lockdown, posting videos and images of crowded quarantine centres and also issuing calls for help with medical treatment and purchasing food. Business has also been disrupted. Volkswagen said it would partially shut production at its Shanghai factory on Thursday, due to a lack of parts from suppliers.

Shanghai government official Ma acknowledged that the city could have done more, and that authorities were trying to improve provision of food supplies as well as addressing issues with seeking medical help.

"Our knowledge about the highly contagious Omicron variant has been insufficient, we were inadequately prepared for the fast-rising number of infected patients, and our control measures have not been up to speed," he said.

Still, despite the irritation among some in Shanghai who question its practicality, the "dynamic clearance" approach adopted by China will not be relaxed, state news agency Xinhua said in a commentary late on Wednesday. The tactic was essential to protect lives and the people's health, Xinhua said.

In Shanghai the city government said it would try its best to prevent the pandemic from spreading further.

"We implore residents to keep cooperating and follow the pandemic control and prevention guidelines. We can only safeguard our home and our city through everyone's efforts."