Authorities in the Yangtze River Delta have activated emergency response systems to combat the latest round of COVID-19 cases found in major cities in the region in the past few days.

In Zhejiang province, 41 people tested positive for novel coronavirus infection between Sunday and 3 pm on Wednesday, with 18 in Ningbo's Zhenhai district, 18 in Shaoxing's Shangyu district, and five in Hangzhou, the provincial capital, a news conference was told on Wednesday afternoon.

All patients were diagnosed with mild COVID-19 symptoms caused by the highly transmissible Delta variant.

The province has conducted nucleic acid tests on 1.33 million residents in the three cities.

In Zhenhai, where the first case was reported on Sunday, a first round of nucleic acid tests on all residents has been completed, with a second round starting at 8 am on Wednesday.

All those infected in Ningbo were from Zhenhai's Jiaochuan subdistrict, where the first confirmed case lived and worked. Closed-loop management was imposed on the area on Monday.

The first confirmed case in the city was a 41-year-old man surnamed Chen who took a high-speed train to Shanghai on Nov 22 and returned to Ningbo the same day.

The city health authorities said five other confirmed cases had received physical therapy at the same moxibustion house in Jiaochuan.

Ningbo upgraded its emergency response to the highest level and a temporary closure of Zhenhai district began early on Tuesday. Schools and kindergartens in the district have been shut down.

Over 4,000 medical workers have aided mass nucleic acid testing work in Zhenhai in the past two days.

In Hangzhou, two confirmed COVID-19 cases and three asymptomatic carriers had been reported by Wednesday morning.

Epidemiological investigation has revealed the outbreaks in Shaoxing and Hangzhou are closely related to each other, but not to the one in Zhenhai, said Zhang Ping, director of the Zhejiang Provincial Health Commission.

Five areas across the province have been identified as medium-risk areas for COVID-19.

In neighboring Shanghai, a residential neighborhood in Pudong district has been designated a medium-risk area and locked down after a resident was found to be infected with novel coronavirus, the municipal government announced on Wednesday.

Six locally transmitted confirmed COVID-19 cases and two asymptomatic cases have been reported in Shanghai over the past two weeks.

All the confirmed cases had traveled outside Shanghai.

The latest case is a 30-year-old man who tested positive on Tuesday after Shanghai authorities acceded to a request from another city to locate the close contacts of a confirmed case.

Nanjing, capital of Jiangsu province, reported one locally transmitted COVID-19 case on Wednesday.

The patient, surnamed Lu, was a contact of a confirmed case reported in another province, the city's COVID-19 joint control headquarters said. She tested positive two days after being released from quarantine and was sent to a designated hospital for treatment on Wednesday.

"Thirty close contacts and 105 sub-close contacts have been initially identified as of Wednesday morning and will all be quarantined," said Xia Haiming, deputy director of Nanjing's health commission.

"All the samples collected from her living environment and contacts had tested negative as of Wednesday morning," he said, adding that two residential communities the woman visited had been put on lockdown.

Vice-Premier Sun Chunlan said on Tuesday that regions across the country should work together and share information in a timely manner to block the transmission chain of the virus and stop its spread.

To reduce the risk of imported infections, all related departments should improve the management of inbound personnel and imported goods, she said at a teleconference held by the State Council interagency task force for COVID-19 response.

Yu Yin in Ningbo contributed to this story.