BEIJING (AP) — China’s new COVID-19 cases Tuesday more than doubled from the previous day as the country faces by far its biggest outbreak since the early days of the pandemic.

The National Health Commission said 3,507 new locally spread cases had been identified in the latest 24-hour period, up from 1,337 a day earlier.

A fast-spreading variant known as “stealth omicron” is testing China’s zero-tolerance strategy, which had kept the virus at bay since the deadly initial outbreak in the city of Wuhan in early 2020. China has recorded more than 10,000 cases in the first two weeks of March, far exceeding previous flare-ups.

No new deaths have been reported in the multiple outbreaks across China, and the case count remains low compared to many other places in the world. The U.K. recorded more than 444,000 cases in the past week. Hong Kong, a semi-autonomous city that tracks its outbreak separately from the mainland, reported 26,908 new cases on Monday alone.

Nearly three-fourths of China’s new infections were in Jilin, a province in the northeast that reported 2,601 cases. Smaller outbreaks have hit more than a dozen provinces and major cities including Beijing and Shanghai.

Jilin has barred residents from leaving the province and from traveling between cities within it. The 9 million residents of Changchun, the provincial capital and an auto manufacturing hub, have been locked down since Friday as authorities conduct repeated rounds of mass testing both there and in the city of Jilin.

More than 1,000 medical workers have been flown in from other provinces along with pandemic response supplies, and the province has mobilized 7,000 military reservists to help with the response.

Elsewhere in China, Shandong province had the most new cases with 106. Guangdong province in the southeast, where the metropolis and major tech center of Shenzhen has been locked down since Sunday, reported 48 new cases. Shanghai had nine, and Beijing, six.