At an office building in China's capital on Sunday, Covid control personnel lugged boxes of pillows and bedding through the closely guarded entrance for workers stuck inside, preparing for what may be days of lockdown as Beijing rushes to prevent the spread of Omicron ahead of the Winter Olympics.

The building in the west of the city has been sealed off, with everybody inside subject to compulsory mass Covid testing, since an employee tested positive for Omicron on Saturday -- the city's first recorded case of the highly transmissible variant.

For the past week, officials in Beijing had been on high alert as an Omicron outbreak spread in Tianjin, a major port city just 30 minutes away by high-speed rail. The cluster had already spread to two other cities hundreds of miles away.