A highly transmissible Omicron subvariant, which is already dominant in Britain and the US, has sent parts of the ancient Chinese city of Xi’an, home to 13 million, into a seven-day lockdown.

Businesses, schools and restaurants in Xi’an will close for one week, officials said on Tuesday, after the Chinese city logged a handful of Covid-19 cases. The capital city of Shaanxi province has reported 18 cases since Saturday in a cluster driven by the fast-spreading Omicron variant, according to official notices.

Some residents in the “high-risk areas” were told to stay at home.

Zhang Yi, of the centre for disease control and prevention in Shaanxi, told local media that the Omicron subvariant, known as BA.5, was behind the latest outbreak in Xi’an.

“The Omicron BA.5 subvariant is even more transmissible and spreads more quickly than the previous BA2.2 sublineage, and it is more likely to escape antibodies,” Zhang told the Chinese media on Tuesday.

The discovery of BA.5 adds new complications to China’s stringent zero-Covid policy, experts say. The subvariant of Omicron is more transmissible than BA.2.2, which were behind recent outbreaks in Shanghai and Beijing.

China is the last major economy wedded to a zero-Covid strategy, deploying snap lockdowns, quarantines and travel curbs in a bid to weed out new infections.

City official Zhang Xuedong said at a press conference that Xi’an would implement “seven-day temporary control measures” that would “allow society to quieten down as much as possible, reduce mobility … and cut the risk of cross-infection”.

“We must race against both time and the virus … to guard against all possible risks and hidden dangers, and decisively avoid an explosion in community spreading,” Zhang said.

Public entertainment venues including pubs, internet cafes and karaoke bars will shut their doors from midnight on Wednesday, the city government said in a notice.

Restaurants will not be allowed to serve diners indoors but may continue to offer takeaway services, it said.

Schools are to start the summer holiday early and universities will seal off their campuses.

Xi’an – home to the Terracotta Warriors – previously experienced one of China’s longest stay-at-home orders, shutting off for a month between December and January as thousands of Covid cases were detected.

City authorities came under fire for their handling of the lockdown, which was plagued by food supply issues and medical tragedies stemming from patients being denied access to hospitals.

Some residents expressed dismay at the closures on social media.

“It’s like they’re addicted to lockdowns. What else do they even do?” wrote one on the Twitter-like Weibo platform.

“Here we go again,” complained another.

China logged 335 new domestic cases on Tuesday, most of which were asymptomatic, according to the national health commission (NHC).

In Shanghai, the city officials announced on Tuesday two new rounds of mass Covid testing of most of its 25 million residents over a three-day period, citing the need to trace infections linked to an outbreak at a karaoke lounge.

“No household or person should be missed,” the city government said of the testing.

Meanwhile, in central Anhui province, 1.7 million people in two counties were under orders to stay at home as of Tuesday.