The authorities said the real number of infections could already be in the hundreds with the virus spreading fast in the densely-populated country.
'Calm before storm'
On Monday, the WHO warned that the flu virus was expected to continue spreading quickly around the globe.
"For the first time in humanity, we are seeing, or we may be seeing, pandemic influenza evolving in front of our eyes," Margaret Chan, the WHO director-general, told ministers and experts in a meeting in Geneva on Monday.
The UN agency has yet to declare a full-fledged swine flu pandemic, but cautioned member states that they may be facing the "calm before the storm".
"[If] this is the calm before the storm [there is] every reason to be concerned with the interaction with other viruses," Chan said.
Seventy-four people have died, mostly in Mexico, the rest in the US, Canada and Costa Rica
Forty countries have confirmed about 8,800 cases
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
"We need to warn the public whenever necessary but reassure them whenever possible. This is a difficult balancing act."
So far 40 countries have confirmed some 8,800 cases of the new flu strain, and most of those infected had relatively mild symptoms.
Almost all of the 74 deaths have been recorded in Mexico, the epicentre of the outbreak.
Chile reported three new cases on Monday, adding to its first two cases reported a day earlier. And tests confirmed the first case in Greece, while New York City recorded its first death from the virus, prompting health officials to shut 16 schools due to outbreaks.
US health officials also expressed concern about who the H1N1 virus was infecting - mostly children, teens and young adults – as well as outbreaks in schools.
Anne Schuchat of the US Centres for Disease Control and Prevention said most of the 200 or so Americans hospitalised with the strain were young, as opposed to the elderly who are usually hardest hit by seasonal flu.
"That's very unusual, to have so many people under 20 to require hospitalisation, and some of them in [intensive care units]," Schuchat said.
"We are also seeing outbreaks in schools, which is extremely unusual for this time of year."
Alert level maintained
Last month the WHO raised its global pandemic alert level to five on a six-point scale.
|The bulk of confirmed H1N1 cases in Japan are not connected to travel [EPA]
It was not clear whether the latest situation in the outbreak in Japan, the largest outside the Americas, would trigger a move to level six.
Under the WHO rules, signs that the disease is spreading in a sustained way in a second region of the world would prompt a declaration that a full pandemic is under way.
Besides the Americas, other large clusters have been seen in Spain and Britain.
A WHO designation of a full pandemic would put countries on even higher alert about the flu strain and give more impetus to pharmaceutical efforts to fight it.
Chan also raised the spectre of H1N1 mixing with the H5N1 avian flu virus entrenched in poultry in several countries.
"No one can say how this avian virus will behave when pressured by large numbers of people infected with the new H1N1 virus," she said.