The northeastern port city of Dalian in Liaoning province reported nine locally transmitted COVID-19 cases and eight asymptomatic infections on Monday, local health officials said at a news conference on Tuesday.

"They were all identified during active screening of key populations in areas under control," said Zhao Lian, deputy director of the city's health commission.

On Monday, after receiving a letter requesting assistance in investigation of two positive cases, local health authorities in Dalian immediately expanded the scope of screening for those at risk and found the nucleic acid test results of 16 mixed samples were abnormal.

After rechecking, the results of the 17 cases mentioned above were shown to be positive.

Zhao said large-scale nucleic acid testing was initiated in Jinpu New Area and Pulandian district.

At present, 1.72 million samples have been taken, and 90 mixed samples have been found to be suspiciously positive, she said.

"Preliminary analysis show that the source of the virus is different from that of other regions in China," said Wang Jie, deputy director of the administrative committee of Jinpu New Area, China's 10th State-level new area.

"The source cannot be ruled out as related to international cargo transport. And there's nothing to suggest it has a connection with cold chain logistics," Wang said.

On Monday, nine subdistricts in Jinpu New Area were put under strict management. They have prohibited all vehicles from entering or leaving except those necessary for emergency, people's livelihoods and epidemic prevention and control.

On Tuesday, Dalian's COVID-19 prevention and control headquarters said six residential units in a neighborhood named Zhen'ai in the Sanshilipu subdistrict of Jinpu were listed as medium-risk.

Since March 15, the city's primary and middle schools were closed and students were learning via online classrooms. In addition, all after-school training activities in Dalian have been suspended.