UNITED NATIONS, March 11 (Xinhua) -- UN Secretary-General Antonio Guterres on Wednesday called for action on the novel coronavirus that causes COVID-19, after the World Health Organization declared the disease a pandemic.

"Today's declaration of a pandemic is a call to action -- for everyone, everywhere. It's also a call for responsibility and solidarity as nations united and as people united," said Guterres in a message.

But he warned against panic. "As we fight the virus, we cannot let fear go viral."

The World Health Organization declared the novel coronavirus a pandemic on Wednesday as it spreads in more than 100 countries around the world.

The WHO defines a pandemic as "the worldwide spread of a new disease." The determination is based on the geographic spread of a disease, the severity of illnesses it causes, and its effects on society.

COVID-19, the illness caused by the new coronavirus, has sickened more than 121,000 people and killed over 4,300 people.

"WHO has been assessing this outbreak around the clock, and we're deeply concerned both by the alarming levels of spread and severity and by the alarming levels of inaction," the WHO's director-general, Tedros Adhanom Ghebreyesus, told reporters in Geneva.