WHO chief Dr Tedros Adhanom Ghebreyesus said the number of cases outside China had increased 13-fold over the past two weeks.
He said he was “deeply concerned” by “alarming levels of inaction” over the virus.
A pandemic is a disease that is spreading in multiple countries around the world at the same time.
However, Dr Tedros said it was still not too late for governments to have an impact. “All countries can change the course of this pandemic,” he said.
Governments had to “strike a fine balance between protecting health, minimising disruption and respecting human life”.
“We’re in this together to do the right things with calm and protect the citizens of the world. It’s doable,” he said.