Shanghai authorities battling an outbreak of COVID-19 have erected mesh barriers outside some residential buildings, sparking fresh public outcry over a lockdown that has forced much of the city's 25 million people to stay home.

Images of white hazmat suit-clad workers sealing entrances of housing blocks and even closing off entire streets with roughly two metre-tall green fencing went viral on social media on Saturday, prompting questions and complaints from residents.

"Isn't this a fire hazard?," said one user on social media platform Weibo.

"This is so disrespectful of the rights of the people inside, using metal barriers to enclose them like domestic animals," said another.

The Shanghai government did not respond to a request for comment.

Most of the barriers appeared to have been erected around compounds designated as "sealed areas", which are buildings where at least one person has tested positive for COVID-19 and so whose residents are forbidden from leaving their front doors.

Reuters was not able to verify the authenticity of all of the photos and videos.

Shanghai, China's largest city and most important economic hub, is battling the country's biggest-ever COVID-19 outbreak with a policy that forces all positive cases into central quarantine facilities.

The lockdown, which for many residents has lasted over three weeks, has fuelled frustration over difficulties accessing food and medical care as well as over lost wages, family separation, conditions at quarantine centres, and censorship of efforts to vent online.

It has also exacted a toll on the world's second-largest economy, with factory efforts to resume production disrupted by snarled supply chains and difficulties faced by locked-down residents returning to work.

The city reported 39 new deaths from COVID-19 for April 23, versus 12 the previous day and by far the most during the current outbreak.

It did not report any deaths from COVID-19 during the first few weeks of its latest case surge, fuelling doubt among residents about the figures. It has since reported 87 fatalities from COVID-19, all in the past seven days.

New case numbers ticked down slightly for Saturday, but remained in the tens of thousands. Shanghai recorded 19,657 new local asymptomatic cases, versus 20,634 a day earlier, and 1,401 symptomatic cases, down from 2,736.

China largely succeeded in keeping COVID-19 at bay following the initial outbreak in Wuhan in late 2019, with a "dynamic zero" policy aimed at stamping out all chains of infection.

That approach has been increasingly challenged by the spread of the highly infectious but less deadly Omicron variant, which has led numerous cities to impose various levels of movement restrictions that have been a further drag on the economy.

Nationwide, China reported 20,285 new asymptomatic coronavirus cases for April 23, versus 21,423 a day earlier, with 1,580 symptomatic cases, down from 2,988.

The capital Beijing recorded 22 new COVID-19 cases - all locally transmitted - compared to six the day before.

(Reporting by Brenda Goh, Norihiko Shirouzu and David Stanway; Editing by Tony Munroe and Christopher Cushing)