China has raised its coronavirus alert level amid a surge in new confirmed cases of the infection.
Beijing' top official Cai Qi told a briefing that the resurgence in Covid-19 cases "has truly rung an alarm bell".
More than 1,000 flights to and from the capital’s two major airports have been suspended.
The ruling Communist Party in China declared victory over the virus in March.
Beijing had essentially eradicated local transmissions until recent days - but has now recorded 137 new cases since last week.
On Tuesday, it was reported that at least one patient is in a critical condition and two are in serious condition.
Four cases were also reported in neighbouring Hebei province, with three linked to the Beijing outbreak.
Most of the cases have been linked to a wholesale food market in the city.
A mass testing campaign targeting anyone who had visited the market in the past two weeks has been launched.
About 9,000 workers at the market had been tested previously.
Beijing now looks to be heading back into lockdown after raising the alert level from three to two.
This change in threat level is expected to bring the cancellation of classes, a suspension of reopenings and stronger requirements for social distancing.
The wholesale food market has been closed for disinfection, as has a second market where three cases were confirmed.
Some residential communities around both markets have been placed under lockdown, so far affecting 90,000 people in a city of 20 million.
The resurgence in China highlights public health experts’ calls for vigilance as many nations move forward with easing virus restrictions to revive their economies.
