The city at the center of China’s worst Covid-19 outbreak in almost two years has tightened lockdown measures and launched another round of mandatory testing for its 13m residents in an effort to extinguish the virus.

Health authorities in Xi’an, the provincial capital of central Shaanxi province, said 150 locally transmitted cases had been discovered on Sunday, down slightly from Saturday. But they said they expected the number of infections to rise as a fourth round of testing for the entire population kicked off on Monday.

Zhang Yi, director of Shaanxi province’s center for disease control and prevention, told state media that Xi’an residents should not be “overly worried about this short-term rapid rise” in daily cases, which have doubled since 75 were discovered on Friday.

“The more cases detected through testing, the better,” Zhang added. “The epidemic should generally slow down with the implementation of control and prevention measures. ‘

The total number of new cases reported nationwide on Sunday reached 162, the highest daily total since April 2020, when China was getting to grips with the devastating pandemic that erupted in Wuhan that January.

The number of cases is minuscule compared with totals in countries such as the US, which reported more than 265,000 new Covid infections and 3,300 deaths on Thursday. But the outbreak in Xi’an is being treated as a crisis in the context of China’s “zero-Covid” epidemic control strategy.

Central government officials are worried that a nationwide outbreak could lead to an overwhelming number of hospitalizations and deaths given the relatively low efficacy of Chinese vaccines, especially as the highly infectious Omicron variant begins to seep into the country.

China’s second Omicron case was reported on December 14 after the carrier, who had returned from overseas, had tested negative during a two-week quarantine in Shanghai and took a domestic flight to his home city of

Guangzhou, in southern Guangdong province.