The recent COVID-19 outbreak in Hunan province has been traced to three tourists from outside the province who traveled to the city of Zhangjiajie in Hunan via Nanjing Lukou International Airport in Jiangsu's provincial capital.

The majority of recent cases around China have been linked to the airport in Nanjing.

The three tourists arrived in Zhangjiajie on July 17 after passing through the airport. Zhangjiajie, a famous tourist attraction in Hunan, has since been linked to infections in several cities including Wuhan, Hubei province, and Beijing.

They stayed in Zhangjiajie until July 23, Gao Lidong, director of the Hunan Provincial Center for Disease Control and Prevention, said on Wednesday. They then went to Hunan's Xiangxi Tujia and Miao autonomous prefecture for a day.

A tour guide in Zhangjiajie and some tourists from Beijing and Chengdu, Sichuan province, got infected in Zhangjiajie while taking part in activities that included having breakfast in hotels, Gao said.

The tourists from Chengdu who were infected in Zhangjiajie transmitted the virus to several people who shared the same sightseeing boat in Changde, Hunan, on July 24.

"Genome sequencing analysis shows all patients in Hunan are infected with the delta variant, which is highly homologous with the outbreak in Jiangsu province," Gao said.

Hunan has reported 45 confirmed locally transmitted cases and 20 asymptomatic carriers in the past week in seven cities, the provincial health commission said.

Beijing reported three new locally transmitted confirmed COVID-19 cases on Wednesday, two of them relatives of a previous confirmed case who had been to Zhangjiajie.

The early cases in the recent outbreak are of the delta variant and are all related to the same transmission chain, Pang Xinghuo, deputy director of the Beijing Center for Disease Control and Prevention, said on Wednesday.

Hubei province has reported 15 confirmed locally transmitted cases, including 12 in Wuhan, the provincial capital. All patients in the province are linked to a tour group from Huai'an, Jiangsu, who had visited Zhangjiajie.

All the patients in Wuhan are from one construction site. One of them came into contact with the Huaian tour group in the waiting hall of Jingzhou Railway Station in Jingzhou, Hubei, on July 27.

Wuhan authorities launched citywide nucleic acid testing on Tuesday to block possible sources of infection and rule out hidden dangers.

"We started nucleic acid testing on all residents in the community at 8 pm and didn't call it a day until 2:45 am the next day," said Wang Cheng, a volunteered aiding testing work at a residential community.

While waiting in line for testing, Wuhan resident Guo Shouqin said: "There were already hundreds of people queuing for testing before I came here at 8 am. People wait in line in order although it's very hot."