Wuhan, Hubei province, has conducted nucleic acid testing for 722,600 residents after the city reported a cluster of COVID-19 infections originating from a health company's training course on Monday, a local health official said on Wednesday.

Meanwhile, 663 close contacts and another 3,018 secondary contacts were isolated in an effort to find other patients and asymptomatic carriers and curb the spread of the coronavirus, said Peng Houpeng, deputy director of the Wuhan health commission.

"The cluster infection resulted in another nine people testing positive for COVID-19 in Wuhan in the 24 hours ending on Wednesday at noon, bringing the city's positive cases to 23 since Monday," Peng said at a news conference in Wuhan on Wednesday.

"Relevant departments are now going all out to block the spread of the coronavirus in local communities," he said.

Of the newly detected positive people, seven attended a training course held in Wuhan between Friday and Sunday, while the others are close contacts, Peng said.

All have been sent to the city's Jinyintan Hospital for further medical observation and treatment.

Peng urged residents not to leave Wuhan unless absolutely necessary to reduce the risk of spreading the coronavirus to other cities.

"Those who must leave are required to show negative results of a test done within 48 hours," he said.

Peng said the training course was held in a hotel in the city's Wuchang district and attended by 66 people. Of those, 24 had left the city for home.