BEIJING, China (AFP)— Tens of millions of people were under lockdown across China on Tuesday, as surging virus cases prompted the return of mass tests and hazmat-suited health officials to streets on a scale not seen since the start of the pandemic.

China reported 5,280 new COVID-19 cases on Tuesday, more than double the previous day's tally, as the highly transmissible Omicron variant spread across a country that has stuck tightly to a zero-COVID strategy.

That approach, which pivots on hard localised lockdowns and has left China virtually cut off from the outside world for two years, appears stretched to the limit as Omicron finds its way into communities.

At least 13 cities nationwide were fully locked down as of Tuesday, and several others had partial lockdowns, with some 15,000 infections reported nationwide in March.

Health officials urged people over 60 to get vaccinated -- including the third booster jab -- as soon as possible.

Around 80 per cent of people in that age group are double-vaccinated, according to official data -- but Beijing is anxiously watching the situation in Hong Kong, which now has the world's highest virus death rates due to low inoculation among its oldest residents. 

Official Jiao Yahui said at a press briefing Tuesday that "the risk of severe illness is very high" for people in that age group.

- 'I panicked' -

The northeastern province of Jilin has been worst hit by Omicron with over 3,000 new cases on Tuesday, according to the National Health Commission.

Residents of several cities there including provincial capital Changchun -- home to nine million people -- are under stay-at-home orders.

Health officials said over 8,200 Jilin residents have been hospitalised, with the vast majority showing mild or no symptoms.

Shenzhen -- the southern tech hub of 17.5 million people -- is three days into a lockdown with many factories closed and supermarket shelves emptying, while China's largest city Shanghai is under a lattice of restrictions.

City officials said at a Tuesday press conference that "it is not necessary to lock down Shanghai at present", instead opting for more "precise" measures.

Scenes of closed neighbourhoods, panic buying and police cordons cast back to the early phase of the pandemic, which first emerged in China in late 2019. 

Although cases from the chaotic initial outbreak in early 2020 are widely believed to have been under-reported, life since then had largely returned to normal in China under its strict zero-COVID approach.

But as lockdowns edge closer to Beijing, public venues have tightened their scrutiny of ubiquitous health QR codes.

Experts forecast a dent to economic growth as the virus billows out.

"Renewed restrictions, notably the lockdown in Shenzhen, will weigh on consumption and cause supply disruptions in the near term," Tommy Wu of Oxford Economics said in a briefing note.

He added that it will be "challenging" for China to meet its GDP growth target for the year of around 5.5 per cent.

Dozens of domestic flights were cancelled Tuesday, and aviation authorities said more than 100 international flights bound for Shanghai would be diverted to other Chinese cities between next week and May 1.

Fears over shutdowns extended a tech-fuelled rout in Hong Kong, where stocks plunged more than six per cent.

As the financial hub grapples with its own deadly COVID-19 wave, a former top government adviser condemned leader Carrie Lam Tuesday and called for her to "resign in shame".

Hong Kong's 4,300 deaths in under three months -- mostly in care homes -- has seen Lam's administration criticised for low vaccination rates and unclear messaging.