Yangzhou, Jiangsu province, reported six new locally transmitted cases of COVID-19 on Tuesday, the Jiangsu Health Commission said on Wednesday morning.

The city has seen 561 locally transmitted cases since July 20, with 26 patients in serious condition and 16 in critical condition still receiving treatment in the hospital.

Wu Zhenglong, Jiangsu's governor, said a risk of virus transmission outside the city's quarantined areas exists.

"The virus containment situation is still very complex," he said. "We must be more responsible, gather more resources from the whole province and become more confident in defeating the virus."

He called for more scientific prevention and control measures and implementation of containment protocols, including improved organization of testing sites, quick tracing of contacts and strict management of medical facilities.

Nanjing, the provincial capital, has reported no new cases for five consecutive days.

Huai'an, another city in Jiangsu, reported 12 cases since July 20. It said college students can return to their campuses on Sept 15. Children in kindergarten to third grade can start their new semester on Sept 9. Those in other grades or who are enrolled in vocational and special education schools can start on Sept 7.