Barack Obama, the US president, said that officials are closely monitoring the cases of swine flu in the country.
"This is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm," he said on Monday.
The new H1N1 virus, apparently a mix of human, pig and bird viruses, is suspected to have killed 103 people in Mexico, but no deaths have been recorded outside the country.
Spain was the first nation in Europe to confirm cases of the virus, with UK confirming two cases on Monday.
People are also being tested for suspected cases in France, Italy, Sweden and Switzerland.
Other suspected cases are being investigated in Brazil, New Zealand and Israel.
Russia, Hong Kong and Taiwan have said travellers returning from flu-affected areas who displayed symptoms of fever would be quarantined.
Several countries have also banned imports of pork and pork products from Mexico and the three US states reporting cases of swine flu.
New Zealand, Israel and several other countries have implemented new security measures at airports and are putting sick visitors under observation.
WHO currently has an alert level of three for the virus, which means the animal flu does not spread well between humans.
|Mexico City residents have been given surgical masks and urged to stay home [AFP]
If raised to four or five, it will mean the virus is becoming adept at spreading between humans, and could also mean further travel advisories, restricted trade and cancelled sporting and entertainment events.
On Sunday, Felipe Calderon, the Mexican president, urged citizens to wash their hands, stay home and avoid crowded places.
The government has declared an emergency in the country, giving it powers to isolate people, enter homes or workplaces and regulate air, sea and land transportation to try to stop further infection.
Peter Cordingley, WHO's regional representative in Manila, the Philippines, said the organisation believes the virus cannot be contained at a local level.
"We've seen already that it has spread to the United States. Whether it can be contained globally we don't know because we don't know at this stage whether it is going to move globally, although there are good reasons to believe that there is a potential for a pandemic," he told Al Jazeera.
"Even with avian influenza, when it seemed to stall and it lost the interest of the public, we continued to talk about the pandemic potential. What we are seeing now is a situation that has accelerated far, far faster than whatever happened with avian influenza. We are seriously concerned."
Richard Besser, the acting head of the US Centres for Disease Control (CDC), told a White House press conference that "we expect to see more cases of swine flu".
"We're responding aggressively to try and learn more about this outbreak," he said.
In Canada, health officials confirmed four cases in Nova Scotia, on the east coast.
In New Zealand, a teacher and 13 students from a group of 25 who just returned from a three-week trip to Mexico, were showing symptoms, health officials said on Monday.
The remaining members of the group were being treated at home and remained in isolation, the health ministry said.
Margaret Chan, the WHO chief, warned on Saturday that the "virus has clearly a pandemic potential", while Keiji Fukuda, the acting WHO assistant-general for health, security and the environment, said the virus could mutate into a "more dangerous strain".
But Fukuda said that if there were such an escalation, the world was better prepared now than it has been in the past.
After the outbreak of Severe Acute Respiratory Syndrome (Sars) in 2003, which killed nearly 800 people, and amid regular reported cases of bird flu, countries have stepped up preparations against a pandemic, including stockpiling anti-viral drugs.