China’s COVID-19 cases have more than doubled in a day, as it faces by far its biggest outbreak since the early days of the pandemic.

The National Health Commission on Tuesday said 3507 locally spread cases had been identified in the latest 24-hour period, up from 1337 a day earlier.

A fast-spreading variant of Omicron, the so-called stealth variant, is testing China’s zero-tolerance strategy. It has previously kept the virus at bay after a deadly initial outbreak in early 2020.

Most of the latest cases were in north-east China’s Jilin province, where 2601 were reported.

But smaller clusters have also broken out across the country, including in the major cities of Beijing, Shanghai and Shenzhen.