Asymptomatic cases of the novel coronavirus have been found in many places in China, which have increased difficulties in the prevention and control of the virus, a senior expert in infectious diseases said on Tuesday.

Medical workers have identified people who tested positive for the virus but have not displayed symptoms among close contacts of patients, Li Xingwang, chief expert in infectious diseases in Beijing Ditan Hospital, said at a news conference.

Some patients, though, just show slight symptoms such as occasional coughing, but they can also be infectious, he said.

Most asymptomatic people infected with the virus are close contacts of patients, and all people who have had close contact with patients have been required to stay under medical observation in isolation to prevent spread of the virus, he said.