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.