China has posted a steep jump in daily Covid-19 infections with new cases more than doubling from a day earlier to a two-year high as a virus outbreak expanded rapidly in the north-east.

A total of 3,507 domestically transmitted cases with confirmed symptoms were reported on Monday across more than a dozen provinces and municipalities, the National Health Commission said, up from 1,337 a day earlier.

Since the coronavirus first emerged in the central city of Wuhan in late 2019, China had successfully suppressed large-scale outbreaks through its strict “zero-Covid” strategy, which involved hard lockdowns that confined huge sections of the population to their homes.

While the country’s case load is still tiny by global standards, health experts said the rate of increase in daily cases over the next few weeks would be a crucial factor in determining whether its tough containment approach was still effective against the rapidly spreading Omicron variant.

A Covid-19 forecasting system run by Lanzhou University in China’s nort-west predicted that the current round of infections would eventually be brought under control in early April after an accumulated total of about 35,000 cases.

The university said in its latest assessment published on Monday that while the outbreak was the most serious on the mainland since Wuhan in 2020, China could bring it under control as long as stringent curbs remained in place.

At least 11 cities and counties nationwide have been locked down because of the latest surge, including the southern tech hub of Shenzhen, home to 17 million people.

In the financial hub of Shanghai, authorities battling an outbreak across the city were cordoning off individual apartment buildings and testing all residents.

China’s aviation regulator said that 106 international flights scheduled to arrive in Shanghai would be diverted to other domestic cities from 21 March to 1 May due to Covid.

Confirmed symptomatic infections in Shanghai stood at 21 on Monday, including 12 imported from overseas, with an additional 130 asymptomatic cases.

Nearly 90% of the mainland’s confirmed new symptomatic cases on Monday were found in the north-eastern province of Jilin, which has banned its 24.1 million population from travelling in and out of the province and across different areas within the province without notifying local police.

Jilin officials should step up the preparation of temporary hospitals and designated hospitals and make use of idle venues to ensure all infections and their close contacts were isolated, a local Communist party authority-backed paper said, citing the provincial head of the party.

An outbreak at Volkswagen Group factories in the Jilin city of Changchun also prompted three sites to shut on Monday for at least three days, according to a spokesman.

The number of new asymptomatic cases, which China does not classify as confirmed cases, stood at 1,768 compared with 906 a day earlier. There were no new deaths, leaving the death toll unchanged at 4,636.

As of 14 March, mainland China had reported 120,504 cases with confirmed symptoms, including both local ones and those arriving from outside the mainland.