Beijing's epidemic control and prevention work is still underway, as recent clusters and increasing COVID-19 cases discovered from mass nucleic acid tests have added uncertainty to the ongoing outbreak, a senior official said on Monday.

"Increasing new cases detected at the community level are the major challenges of the control and prevention work in Beijing while the recent cluster infections were all caused in areas outside of the controlled zones," said Xu Hejian, spokesman for the Beijing government, at a news conference on Monday.

Beijing has done 10 rounds of mass nucleic acid tests since the outbreak began on April 22. New cases were found during every single round, he said.

Beijing has started another three rounds of tests in 12 districts from Monday to Wednesday, trying to find potential risks as early as possible.

Recently, Beijing reported four clusters involving a market, a subway renovation project, a bus station and a branch of a logistics company, according to the Beijing CDC on Monday.