"I would still propose that a pandemic is imminent because we are seeing the disease spread," Michael Ryan, WHO director of Global Alert and Response, said.
Costa Rica, Italy, and Ireland confirmed cases of the virus, which has now been found in 18 countries.
WHO said 15 countries had reported 615 infections, not including the later reports of confirmed cases in Mexico, Ireland, Italy and Costa Rica.
Outbreak in Asia
On Friday South Korea confirmed the country's first case of the H1N1 flu, making it the second Asian nation to report an outbreak of the potentially fatal virus.
A 51-year-old woman has been in quarantine since returning from Mexico, Lee Jong-koo, chief of the state disease control centre, told a news conference on Saturday.
Seoul's confirmation came as China quarantined hundreds of people in a Hong Kong hotel and cancelled all flights between the city of Shanghai and Mexico.
A traveller who arrived in Hong Kong from Mexico via Shanghai has tested positive for H1N1 strain of the influenza virus.
All of the 176 passengers and 13 crew from the Mexicana airlines flight which brought the traveller to Shanghai will also be placed under "seven-day medical observation", China's health ministry said.
It called on all passengers from that flight and another which took the Mexican patient from Shanghai, to Hong Kong to get in touch with the health authorities "to ensure that all passengers can get timely medical tests".
Health workers in white bodysuits patrolled the lobby of Metropark Hotel in Hong Kong on Saturday as guests picked up bottles of water, chocolate milk and bread before returning to their rooms.
Police officers wearing masks guarded the building, which was cordoned off with police tape. Mexico called the action "unjustified", but advised its citizens to avoid travel to China for the time being.
A Hong Kong-based infectious diseases specialist criticised the quarantine as an over-reaction and said it served little practical purpose.
"This is only one point in his journey. And it's not the highest risk point in his journey. Flu spreads through coughs or sneezes at close range. People who lived above and below him are nearly not at risk at all," Lo Wing-lok said.
But Donald Tsang, Hong Kong's leader, defended the decision in light of the Sars (Severe Acute Respiratory Syndrome) outbreak in 2003, which killed nearly 300 people in the territory.
"I'd rather err on the side of caution than miss the opportunity to contain the disease," he said.
India was also monitoring two patients suspected of having contracted H1N1 after arriving from abroad, according to doctors.
"Both of them are under observation in an isolation ward. We have done all the tests, and samples have been sent to the National Institute of Communicable Diseases," NK Chaturvedi, medical superintendent of the state-run Ram Manohar Lohia hospital, said.
If confirmed, the cases would be the first in the country of 1.1 billion people, raising fears that it could spread across the densely populated country.
On Friday, Mexico began a five-day shutdown of all but essential government services and private businesses in an attempt to halt the spread of the flu.
'Running its course'
Barack Obama, the US president, voiced hope that the virus may turn out to be no more harmful than the average seasonal flu.
Nineteen confirmed in Mexico out of 473 suspected. One death in the United States
Countries with confirmed cases: Mexico, the US, Canada, New Zealand, Spain, Israel, Britain, Germany, Austria, Switzerland, The Netherlands, China, Denmark, France, South Korea, Italy, Ireland, Costa Rica
Countries with suspected cases: Australia, Brazil, Chile, Colombia, Norway, Guatemala, India
Annual influenza epidemics are thought to result in three to five million cases of severe illness and between 250,000 and 500,000 deaths around the world, WHO says
"It may turn out that H1N1 runs its course like ordinary flus, in which case we will have prepared and we won't need all these preparations," he said.
Obama stressed that the government was still taking the outbreak, which has killed at least one person and affected at least 154 others in the US, very seriously.
British officials said that two of the 15 total patients being treated for H1N1 were infected by human-to-human transmission rather than having become infected in Mexico.
Germany and Spain have also reported such cases.
Italy was the latest European country to confirm a case of the virus, but a regional health official said that the man was already completely recovered.
Nancy Cox, influenza chief at the US Centre for Disease Control and Prevention, said that it was positive that the genetic makeup of the virus did not show specific traits of the flu pandemic virus which killed up to 50 million people worldwide in 1918.
"However, we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses," she told The Associated Press news agency.
The World Health Organisation (WHO) said on Friday that tests had shown that the vaccine against seasonal flu would have little effect against the new H1N1 strain.
The group also said that due to the rapid spread of the virus and the possibility that it could hit poor and disease-prone communities, including people with HIV, the world should remain prepared for a pandemic.