Eighteen people who tested positive for COVID-19 on Sunday in Lhasa, Tibet autonomous region, had the highly contagious transmissible Omicron variant, the city's health authorities said at a news conference on Tuesday.

The first confirmed patient, surnamed Dan, is a 3-year-old who traveled from the region's Shigatse city to Lhasa by train with his mother and grandmother on Thursday.

Seven other people who had close contact with the three also traveled from Shigatse to Lhasa. They lived in a same house in a community compound on Lhasa's Karma Gonsar Street. They came to pay their respect to a deceased relative. The other eight people also lived in the same house.

Yan Zhouzhong, deputy director of the Lhasa health commission, said the outbreak was mainly due to the gathering of people. The current epidemic shows three characteristics, Yan said.

First, the strain is the Omicron variant called BA.2.76. Second, an epidemiological investigation found that the epidemic is imported. Third, it spread at a family group gathering.

"Now, all the 18 infected patients have been hospitalized and are in a stable condition, without any obvious discomfort, and they are in a good psychological condition," Yan said.

The city has seven designated quarantine hotels, with 119 relatives of the first confirmed case put under quarantine to curb the spread of the virus. Ninety other people with close contact with the first confirmed patient and 54 residents of Lhasa also have been isolated in hotels.

The world heritage Potala Palace in Lhasa, announced on Tuesday that it would close to visitors to guard against the new outbreak, according to a statement released on the palace's official website.

As of Monday night, Tibet had two patients diagnosed as confirmed cases and 20 as asymptomatic cases, according to the region's health commission.