The southern Chinese tech hub Shenzhen was put under a citywide lockdown today, the local government announced, after it reported 66 new coronavirus cases and sealed off the central business district.

Home to tech giants including Huawei and Tencent, the city of around 17 million across the border from Covid-hit Hong Kong has told residents to stay home to tamp down an Omicron flare-up.

It had already closed non-essential venues and banned restaurant dining in recent days.

It comes as China reported 3,393 new Covid-19 cases, the National Health Commission said, more than double the previous day, as the country faces its gravest virus outbreak in two years.

A nationwide surge in cases has seen authorities close schools in Shanghai and lock down several northeastern cities, as almost 19 provinces battle local outbreaks of the Omicron and Delta variants.

The region of Jilin has also been partially locked down, with hundreds of neighbourhoods sealed up, an official announced.

Authorities in Changchun, the provincial capital already under lockdown, have been conducting mass testing and working on turning an exhibition centre into a temporary hospital with 1,500 beds, according to the local official and state media reports.

The industrial base of nine million people was locked down on Friday.

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Workers set up a makeshift hospital for Covid-19 patients in Changchun, China

"[The outbreak] reflects that the spread of the Omicron variant is hidden, highly contagious, rapid and difficult to detect in the early stages," said Jilin provincial health official Zhang Yan at a press briefing.

"It also reflects the rapid rise of the virus situation in individual regions and the lack of expansion capacity of medical resources, resulting in limited centralised admission and treatment in a short period of time."

The mayor of Jilin and the head of the Changchun health commission were dismissed from their jobs yesterday, state media reported.

The smaller cities of Siping and Dunhua, both in Jilin province, were locked down Thursday and Friday, according to official announcements.

Hunchun city, bordering Russia and North Korea, was locked down on 1 March, local authorities said.

Three makeshift hospitals were built in the city to treat Covid-19 patients, Xinhua reported yesterday.

China, where the virus was first detected, has maintained a strict 'zero-Covid' policy enforced by swift lockdowns, travel restrictions and mass testing when clusters have emerged.

But the latest flare-up, driven by the highly transmissible Omicron variant and a spike in asymptomatic cases, is challenging that approach.