The World Health Organization said Tuesday that people in their 20s to 40s are driving the spread of the new coronavirus in Asia, with many showing mild to no symptoms of the disease.

Takeshi Kasai, the WHO's Manila-based regional director for the Western Pacific, said during a press conference that many people in these age groups could unknowingly be passing on the virus to others.

"This increases the risk of spillovers to the most vulnerable -- the elderly, the sick people in long-term care, people who live in densely populated urban areas and under-served rural areas," Kasai said, adding that efforts must be redoubled to protect vulnerable communities.

"Those are the age groups that usually, even when you get the infection, you might not develop the obvious symptoms that push you to go to testing," he added.

Kasai estimated that 16 percent of infected people never develop symptoms of COVID-19, the respiratory disease caused by the coronavirus, but may still be able to spread it.

He added that these people can contribute to driving up the number of cases.

More than 400,000 COVID-19 cases have been recorded in the Western Pacific Region, home to almost 1.9 billion people across 37 countries and areas. Those include nearly 9,300 deaths, according to the health organization's latest tally.