Beijing reported a locally confirmed COVID-19 case and one asymptomatic carrier as of 3 pm on Friday. Both had taken a train that departed from Lhasa, Tibet autonomous region, on Monday, according to the city's disease prevention and control authorities.

The train they took — Z22 — terminated at Beijing. At least 29 infected passengers on the train were tested positive for COVID-19 by Friday noon who disembarked at various destinations.

The two in Beijing, one in Shunyi district and another in Daxing district, tested positive for COVID-19 at quarantine centers after they arrived in the capital, Liu Xiaofeng, deputy director of the Beijing Center for Disease Prevention and Control, said at a news conference on Friday.

Before the centralized quarantine, the person in Shunyi spent the night in a minivan for self-quarantine after having been notified to have close contact with infected people on the train. The van was also tested and the results came back negative.

Since Monday, Beijing has detected 12 people positive for COVID-19, four of whom were infected while they were on trains heading to the capital, Liu said.

"Passengers taking trains face increased transmission risk because of the long travel time, the large volume of people and the confined space," Liu said.

He suggested passengers protect themselves by avoiding places that have COVID-19 outbreaks.

Since early August, Beijing has seen seven COVID-19 outbreaks that are related to other regions in the country and two related to outbreaks overseas, said Xu Hejian, speaking for the Beijing municipal government at the news conference. The pressure for preventing imported infections and new local outbreaks have increased, Xu said.