Dr Tedros Adhanom Ghebreyesus's comments follow Iran's announcement of two more deaths, bringing the total there to four.
The window of opportunity to contain the virus was "narrowing", he said.
Iranian health officials said the virus may already be in "all Iran's cities".
Outside China 1,152 cases of the virus have been confirmed in 26 countries and there have been eight deaths.
They include two deaths in South Korea, which has the biggest cluster of confirmed cases apart from China and a cruise ship quarantined in Japan.