All 12 million residents of Wuhan, Hubei province, will be tested for novel coronavirus after a new cluster was detected in the Chinese city hardest hit by COVID-19 last year.

Control and prevention measures were reactivated in Wuhan on Tuesday following the confirmation of three locally transmitted cases on Monday. They are the first cases detected in the city since June last year.

Wuhan authorities have decided to launch citywide nucleic acid testing immediately, Li Tao, deputy secretary-general of the city government, said at a news conference.

"Comprehensive screening can effectively block possible sources of infection and eliminate hidden dangers," Li said.

Since Saturday, Hubei has reported six confirmed locally transmitted cases, and eight asymptomatic carriers, according to data from the provincial health authority.

One of the three infected people, surnamed Tang, came into contact with a tour group from Huai'an, Jiangsu province, in the waiting hall of Jingzhou Railway Station in Jingzhou, Hubei, on July 27.