Beijing confirmed a COVID-19 patient on Monday that might have been infected by an imported case from the United Kingdom, an official said on Tuesday.

The infected patient, surnamed Liu, started to suffer symptoms of fever and fatigue on March 10.

Liu, 42, lives upstairs from a returnee from the UK in the Dahezhuangyuan residential community on Suzhou Street, Haidian district, Pang Xinghuo, deputy director of the Beijing Center for Diseases Prevention and Control, said on Tuesday at a daily press conference.

The returnee, surnamed Li, came to Beijing on March 5 and was required to stay at home for self-quarantine, while Li still ventured out for activities in the community and to go to a supermarket.

Li went out through shared residential building stairs on March 7, started to have symptoms on March 9 and walked to Beijing Haidian Hospital on March 11 for treatment. Li was confirmed to have been infected with COVID-19 on March 12.

"Liu, who also used the same stairs, might have had contact with a contaminated environment on the stairs that Li had used before March 11, and thus got infected," Pang said.

Pang warned that the situation of COVID-19 is still severe and complicated. People should continue to take protective measures, including wearing masks and reducing gatherings. Residential building management should constantly disinfect public areas, she added.

To prevent the risk of infections brought by increasing imported cases, the capital has taken a series of upgraded measures in recent days.

Customs in Beijing refused four foreigners entry to the country because they were suspected of violating relevant policies concerning the city's anti-virus efforts, Ji Lixia, deputy general head of the Beijing General Station of Exit and Entry Frontier Inspection, said at the conference.

"Some of the foreigners were reentering the country during a supposed quarantine time after leaving China," Ji said.

For Chinese residents who are under required quarantine, they also will not be allowed to go out of the country, Ji added.

Passengers arriving in Beijing from overseas should report their planned journeys ahead of time to residential communities and companies they work at, read related guidance during trips, and go to designated sites for further arrangements, Chen Bei, deputy secretary-general of the Beijing municipal government, said at the conference.