Beijing,  Authorities in Beijing are on high alert for a surge in coronavirus cases amid a fresh outcry in Shanghai over buildings blockaded under China's zero Covid policy, the media reported.

The number of new cases in the capital rose by 22 on Sunday -- all locally transmitted -- compared with six the day before, according to official reports. Beijing authorities have so far not taken steps to lock down the capital, but they have ordered a number of gyms and after-school activity providers to suspend in-person classes, The Guardian reported.

Residents rushed to stockpile food amid rumours of tougher measures in the coming days.

Health official Pang Xinghuo said preliminary observations suggested Covid had been "spreading invisibly" within the capital for a week, affecting "schools, tour groups and many families".

"The risk of continued and hidden transmission is high, and the situation is grim," Tian Wei of Beijing's municipal party committee told a press briefing. "The whole city of Beijing must act immediately."

The outbreak in Beijing came as Mainland China's most crucial financial hub, Shanghai, enters its fourth week of city-wide lockdown. Thirty-nine new deaths were reported on Sunday, compared with 12 the previous day and by far the most during the current outbreak.

As the situation did not seem to have improved following three weeks of stringent lockdown in Shanghai, desperate officials erected mesh barriers outside some residential buildings on Saturday. This move -- described in the official directive "hard isolation" -- sparked a fresh public outcry over a lockdown that has forced many of the city's 25 million people to stay home, The Guardian reported.

Images of white hazmat suit-clad workers sealing entrances of housing blocks and even closing off entire streets with 2-metre-tall green fencing went viral on social media on Saturday, prompting questions and complaints from residents.