Its chief, Tedros Adhanom Ghebreyesus, said the virus was "unique" but could be contained with the right measures.
Deaths globally have passed 3,000. Most are in China but over the past day there were nine times more new infections outside China than inside.
Dr Tedros insisted: "We can push this virus back". He also said stigma was more dangerous than the disease itself.
The WHO chief said the development of the Covid-19 disease globally was not a "one-way street" and could be combated if countries acted quickly and effectively - starting with containment measures.
"There is no choice but to act now," he said.
One of the countries worst affected outside China - Italy - on Monday saw a jump in its death toll from 34 to 52.
Dr Tedros's main advice was that each country had to look at their own situation as there was no one-way-fits-all means of tackling the outbreak.
"Each country must have its own approach but it must start with containment," he said.
This was a "unique virus with unique features" and the WHO would have expected wide contagion by now, but containment measures had appeared to work, the doctor said.
Of the 62 countries reporting cases of infection, 38 of them had 10 or fewer, he added.
"Around eight countries have not reported new cases for two weeks and have been able to contain the outbreak," he said. China had also shown containment was possible even in countries with a large number of cases.
The WHO would continue to monitor whether the outbreak should be called a global pandemic, Dr Tedros said.
South Korea, Italy, Iran and Japan remain the greatest concern. A WHO staff member has tested positive in Iran, although the symptoms are mild, Dr Tedros said.
He also added: "Stigma, to be honest, is more dangerous than the virus itself. Let's really underline that. Stigma is the most dangerous enemy."
What is the situation globally?
There are now almost 90,000 cases worldwide in about 70 countries, although the vast majority - just under 90% - remain in China, and most of those are in Hubei province, where the virus originated late last year.
Of the nearly 8,800 cases outside China, 81% are in four countries - Iran, South Korea, Italy and Japan.
Italy raised its death toll by 18 on Monday, with 1,835 confirmed cases, most of them in the Lombardy and Veneto areas of the north.
Iran reported another 12 deaths, taking the total there to 66. They included Mohammad Mirmohammadi, a high-ranking adviser to the country's Supreme Leader Ali Khamenei, Iranian media reported.
Health officials in the US state of Washington said on Monday that four more people had died, bringing the total there to six. They are the only deaths in the US so far.