"We're at a decisive point," WHO chief Tedros Adhanom Ghebreyesus told reporters in Geneva.
Tedros said the number of new cases in China had been outstripped by new cases outside the country for a second day running.
"It's what's happening in the rest of the world that's now our greatest concern," he said.
Tedros told affected countries: "If you act aggressively now, you can contain this virus, you can prevent people getting sick, you can save lives."
But he emphasised that all countries should ensure that their health systems were prepared for an outbreak.
Tedros said all health ministers should be asking themselves a series of questions such as whether they had enough ventilators, whether they were ready to carry out tests and whether they could fight misinformation.
"We are actually in a very delicate situation in which the outbreak can go in any direction based on how we handle it... It can be contained, but if we don't do the right thing it can get out of control," Tedros said.
"When you do containment measures like China is doing, you can actually see a decline in the cases and ultimately it can be contained," he added.
Tedros also said 90 percent of cases had a fever and 70 percent had a dry cough and that a runny nose was "not usually" one of the symptoms.
He reiterated concern about the sudden surge in cases in Iran, Italy and South Korea.
"Italy, a member of the G7, it was really a surprise, so many other developed countries should also see some surprises, should expect some surprises," he said.
Iran has a particularly high number of deaths relative to the number of confirmed cases - around 10 percent - compared to a level of around 2 percent elsewhere.
"The most likely factor is that obviously this disease came unseen and undetected into Iran so the extent of infection may be broader than what we are seeing," said Michael Ryan, head of WHO's health emergencies programme.
"As is the case in many epidemics when they start, is that you only see the severe end of the spectrum and then as you do more surveillance, you find more mild cases.
"There is a very high clinical capacity for managing severely ill patients in Iran, so I don't suspect this has anything to do with clinical care. I suspect this is more to do with surveillance and detection of cases," he said.