A staff member processes samples for nucleic acid test at the laboratory of KingMed Diagnostics, a Chinese medical diagnostic testing company in Beijing, capital of China, June 25, 2020. (Xinhua/Zhang Chenlin)

The city of Qingdao in east China's Shandong Province reported three new asymptomatic cases of COVID-19 on Sunday, local health authorities said.

Two of the three cases, two males in their late 50s, tested positive for the virus when going to hospitals for treatment of other diseases, while the third one, a 53-year-old female, was a close contact of a confirmed case, according to the Qingdao Municipal Health Commission.

All the patients have been put under quarantine at a designated hospital.

Epidemiological investigations showed that all the three asymptomatic cases were associated with the Qingdao Chest Hospital, which has set up independent areas for treating imported cases of COVID-19, the commission said.

The specific source of infection is under further investigation.