Some provinces and regions in China have not reported new coronavirus cases for days now, but an expert has warned that zero new cases doesn't mean zero risk, as the possibility of resurgence can't be ruled out.

"We still have a lot of unknowns regarding the novel coronavirus, including the source of infection, virulence and mutation," Liang Wannian, head of the Chinese expert panel on outbreak response and disposal, said during an interview with Chinese broadcaster CCTV on Saturday.

From the disease itself, "We only have a rough idea about the proportion of asymptomatic, mild, severe and critical cases of COVID-19," said Liang. "The whole picture remains unclear. That's why the risk still exists in spite of zero cases (in some provinces and regions)."

Mortality rate of COVID-19 low in China

Data indicates that China has passed the peak of the outbreak, said Liang.

From onset, development to end, the progress of an infectious disease outbreak comprises five phases, which are emergence, rise, peak, decline and post-epidemic stages.

China has gone through the peak of the epidemic and entered the decline stage, Liang said. The outbreak "is already at a low mortality level."

While China's epidemic prevention and control continues to improve, the situation globally is still severe. Preventing cases imported from outside the country has also become a major challenge for China, cautioned Liang.

"It must be fully recognized that risks exist," Liang stressed, adding that the key to containing the virus is to implement early detection, reporting, isolation and treatment.

'Cooperation is a must' for controlling COVID-19 pandemic

The World Health Organization (WHO) has declared the COVID-19 a pandemic, yet Director-General Tedros Adhanom Ghebreyesus noted it would be the first pandemic in history that could be controlled.

The key lies in how countries take measures and strategies to control the further spread on their own soil, said Liang, calling on worldwide cooperation.

"China has been proactive in sharing our experience of prevention and control with the international community. The virus is a common enemy to all mankind, so only by cooperating effectively can we ultimately defeat it," Liang highlighted. "Cooperation is a must."