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.
An evacuation flight carrying 32 British and other European passengers has taken off from Japan and is due to land in England later on Saturday.
On Friday, doctors in Italy said a 78-year-old man became the first person in the country to die from the new coronavirus, Ansa news agency reported.
Earlier in the day Italy announced 16 more cases and its health minister said schools and offices would be closed and sports events cancelled in the affected regions.
China has reported 75,567 cases including 2,239 deaths. The new virus, which originated last year in Hubei province in China, causes a respiratory disease called Covid-19.
What did the WHO chief say?
Dr Tedros said the number of coronavirus cases outside China was "relatively small" but the pattern of infection was worrying.
"We are concerned about the number of cases with no clear epidemiological link, such as travel history to or contact with a confirmed case," he said.
The new deaths and infections in Iran were "very concerning", he said.
But he insisted that the measures China and other countries had put in place meant there was still a "fighting chance" of stopping further spread and called on countries to put more resources into preparing for possible outbreaks.
What about the Iran cases?
In Iran the outbreak is centred on the holy city of Qom, south of the capital Tehran, which is a popular destination for Shia Muslims in the region.
Iran reported two more deaths in Qom on Friday, adding to the two deaths it reported on Thursday. A total of 18 cases have been confirmed in the country.
Lebanon has reported its first confirmed case - a 45-year-old woman who was detected as she arrived in Beirut from Qom. The UAE, Israel and Egypt have also reported cases.
Meanwhile Canadian officials said one of the nine cases there was a woman who had recently returned from Iran.
WHO officials said both Iran and Lebanon had the basic capacity to detect the virus and the WHO was contacting them to offer further assistance.
But Dr Tedros said the organisation was concerned about the virus's possible spread in countries with weaker health systems.