All countries should step up surveillance for unusual outbreaks of influenza-like illness and severe pneumonia, recommends the organisation. It is preparing to raise the alert level to grade four (out of six) signaling that health authorities should prepare for a pandemic.
Stocks of anti-viral medicines are being prepared and travellers are being screened for symptoms of the H1N1 virus at some airports. Human cases usually occur in those who have contact with pigs but, until now, human-to-human transmission has been rare. In humans, the virus causes fever, aches, coughing, a sore throat, respiratory congestion and vomiting.
Russia and Serbia have banned imports of raw pork and pork products from Mexico and the US states of California, Texas and Kansas. Other countries have stepped up screening of pork imports from the affected regions. But, as yet, there is no evidence that eating pork can lead to infection. In response to the outbreak the dollar and Mexico's peso have both weakened while Asian stock markets and the price of oil have fallen.
Health officials are particularly concerned because the new strain seems to target young, healthy adults; which was the same group affected by Spanish flu outbreak of 1918 which killed up to 50m people.
Mexican health authorities noted a sharp rise in flu-like cases last month but attributed it initially to normal winter flu. On April 13th, president Calderón of Mexico ordered the isolation of those infected with the flu strain.
At least 20 cases have been confirmed in the US, leading the government to declare a public health emergency and release 12m doses of the anti-viral drug Tamiflu.
Six cases have been confirmed in Canada and outbreaks are also suspected in the UK, Israel, France, Spain and New Zealand.
The virus H1N1 is the same strain that causes seasonal flu outbreaks in humans but the new version contains genetic material from versions of flu which usually affect pigs and birds.
Although there is no vaccine for the new strain of flu, severe cases can be treated with antiviral medication.
WHO experts will meet in Geneva tomorrow to decide whether to institute the pandemic alert phase.
Meanwhile, the World Bank is providing Mexico with more than $200m in loans to help it deal with the outbreak.