China reported 13,000 Covid cases on Sunday, the most since the peak of the first pandemic wave over two years ago, as health officials said they have found a suspected new subtype of the Omicron variant in the Shanghai area.

China's "zero-Covid" strategy is under extreme pressure as the virus whips across the country.

Until March, China had successfully kept the daily caseload down to double or triple digits, with hard, localised lockdowns, mass testing and travel restrictions.

But cases have surged over recent weeks to thousands each day, especially in the outbreak epicentre of Shanghai, where streets were eerily empty on Sunday as 25 million people stayed in under lockdown orders.

Officials in Suzhou, a city 30 minutes west of Shanghai, have detected a mutation of the Omicron variant not found in local or international databases, state media reported on Sunday.

"This means a new variant of Omicron has been discovered locally," Xinhua said, citing health official Zhang Jun, deputy director of the Suzhou Center for Disease Control and Prevention.

The current outbreak is also testing the patience of the Chinese towards tough restrictions, at a time when much of the world has re-opened.

On Sunday, the 1.5 million residents of Baicheng in northeast China joined the ranks of tens of millions of other Chinese who have endured some form of lockdown over the last month, disrupting work and damaging the economy.

China recorded 13,146 cases on Sunday, the National Health Commission said in a statement, with "no new deaths" reported.

It is the country's highest daily infection tally since mid-February 2020.

The streets of Shanghai were quiet Sunday as a citywide lockdown dragged on, with China reporting 13,000 Covid cases -- the most since the peak of the first pandemic wave over two years ago
The streets of Shanghai were quiet Sunday as a citywide lockdown dragged on, with China reporting 13,000 Covid cases -- the most since the peak of the first pandemic wave over two years ago AFP / Hector RETAMAL

Nearly 70 percent of the national caseload was found in Shanghai, the commission said, after mass testing the metropolis' 25 million residents.

City authorities have conceded they are struggling to contain the outbreak, with thousands now in state quarantine and reports circulating of health workers being stretched.

Vice Premier Sun Chunlan urged "resolute and swift moves" to snuff out the outbreak after a visit to Shanghai, Xinhua reported Sunday.

Anger is rising among residents over lockdowns that were initially planned to last just for four days, but now appear likely to drag on for several more as fresh rounds of mass testing are carried out.

Parents have expressed fears of separation from their children in the event of a positive test, while residents have griped about a lack of fresh food and the ability to walk dogs outside.

China, the country where the coronavirus was first detected in the central city of Wuhan in late 2019, is among the last remaining places following a zero-Covid approach to the pandemic.

The outbreak has taken on an increasingly serious economic dimension, trimming analysts' growth projections as factories close and millions of consumers are ordered indoors.

Shanghai's restrictions threaten to snarl supply chains, with shipping giant Maersk saying some depots in the city remained closed and trucking services would likely be hit further due to the lockdown.

The World Health Organization's emergencies director Michael Ryan last week said it was important for all countries, including China, to have a plan to wind down pandemic restrictions.

But he said China's vast population provides a unique challenge to its health system and authorities will have to "define a strategy that allows them to exit (the pandemic) safely".