The WHO said it was too early to call the outbreak a pandemic but countries should be "in a phase of preparedness".
A pandemic is when an infectious disease spreads easily from person to person in many parts of the world.
More cases of the virus, which causes respiratory disease Covid-19, continue to emerge, with outbreaks in South Korea, Italy and Iran causing concern.
However, most infections are in China, the original source of the virus, where 77,000 people have the disease and nearly 2,600 have died. The number of new cases there is now falling.
More than 1,200 cases have been confirmed in about 30 other countries and there have been more than 20 deaths. Italy reported four more deaths on Monday, raising the total there to seven.
Worldwide stock markets saw sharp falls because of concerns about the economic impact of the virus.
China said it would postpone the annual meeting of the National People's Congress next month, to "continue the efforts" against the coronavirus.
The body, which approves decisions made by the Communist Party, has met every year since 1978.