China reported 140 new cases of coronavirus on Saturday, the highest number in four months, as authorities scramble to contain the outbreak in several regions, including the city of Xi’an, where Millions of people are trapped.

Of the 140 new infections, 87 were transmitted locally, according to a statement from the National Health Commission, up from 55 the day before.

Most of the infections occurred in the city of Xi’an, Shaanxi Province (northwest), where 13 million people have been isolated since Thursday.

China is on high alert as it prepares to host the Winter Olympics in February in the capital, Beijing.

As the state has reduced the number of cases to a minimum thanks to a “zero Covid” strategy that includes border restrictions, lengthy quarantines and targeted closures, cases have increased in recent weeks.

Under strict containment rules, as of Thursday, families in Xi’an are no longer allowed to send one person outside every two days to purchase basic necessities. Residents must obtain special permission from their employer or local authorities to leave the city.