Dr Mike Ryan praised China's response to the deadly outbreak, saying: "The challenge is great but the response has been massive."
The WHO will meet on Thursday to discuss whether the virus constitutes a global health emergency.
The Chinese city of Wuhan is the epicentre of the outbreak.
But the virus has spread across China and to at least 16 countries globally, including Thailand, France, the US and Australia.
More than 130 people have died in China and close to 6,000 have been infected.