A cluster of COVID-19 infections has been reported at a college in Yingkou, Liaoning province, that has swelled to 177 cases since Sunday and triggered stronger prevention efforts and an apology.

The city in Northeast China has reported 186 asymptomatic locally transmitted COVID-19 infections since Sunday. Of those, 177 cases were linked to Yingkou's Liaoning Agricultural Technical College.

On Tuesday afternoon, a letter to all students and their parents was posted on the college's website, apologizing for the sudden outbreak.

It said the college's epidemic prevention and control work is progressing and supplies are adequate. Efforts are being made to ensure the supply of food for the students and other living needs.

According to an article previously posted on the college's website, 69 faculty and service staff members at the college have been stationed permanently on campus to protect the safety and health of 6,848 students since March 13, when the college began closed-loop management.

On Wednesday, Yingkou's Bayuquan district, where the college is located, initiated its second round of mass nucleic acid testing for all residents.

Li Lecheng, governor of Liaoning province, went to Yingkou on Tuesday to coordinate efforts to solve problems on site.

He said efforts should be made to reduce weaknesses in routine prevention and control, to speed up the deployment of routine testing sites and to improve the capacity for early detection.

He also called for better services for the general public, especially students.

In Dandong, another city in Liaoning province, the principal and vice-principal of a middle school are being investigated for negligence because of their poor handling of COVID-19 prevention and control, which led to a cluster of infections recently, the city's commission for discipline inspection and the supervision commission said on Wednesday.

Another cluster of infections has been found and put under control in the city of Zhengzhou.

The capital of Henan province is carrying out large-scale nucleic acid testing in its eight major districts because of an outbreak linked to Zhengzhou East Railway Station.

"The latest outbreak since April 30 has a risk of further spread in the community, with the infected people having visited many public places," Li Huifang, deputy secretary-general of the Zhengzhou government, told a news conference on Tuesday.

Efforts have been made to bring the overall epidemic risk under control within one week, she said.

Li said the COVID-19 infections detected at Zhengzhou East Railway Station, a major railway transfer station in Central China, were found to have been spread by confirmed cases from outside the province who transferred at the station.

Taking April 23 as a time node for investigation, Zhengzhou carried out a flow survey of all relevant cases and the places they visited.

So far, 560 people who had close contacts with the case and 1,522 secondary contacts have been checked.

Since March, Zhengzhou has reported a total of 161 infections, including 31 confirmed cases and 130 asymptomatic carriers, the local health commission said on Tuesday.

City authorities have implemented strict management in its main urban areas for a week starting on Wednesday, suggesting residents not leave unless necessary and for government employees and public servants to work at home.

The epidemic prevention and control situation is grim and complex, but various departments in the city will go all out to block the further spread of COVID-19 to achieve the goal of clearing the community and protecting residents, Li said.

Zheng Caixiong contributed to this story.