The estimated death toll from swine flu in Mexico has risen to 152, officials have said, as fears of a pandemic grow.
"We're in the decisive moment of the crisis. The number (of deaths) will continue rising," said Mexico's health minister Jose Angel Cordova.
He claimed it was too early to identify the cause or geographical source of the virus, which is now thought to have infected close to 2,000 people in the country.
However, Mr Cordova told reporters that a case in the southern state of Oaxaca was the first that had made them aware they were not dealing with a normal seasonal variation of flu.
Mexican authorities have shut schools across the country and are attempting to follow up families of swine flu victims in a bid to stop the spread of the virus.
In the country's popular tourist resorts, precautions are also being taken.
Reporting from Mexico, Sky News producer Clint Stanaway said: "There is some concern about how quickly this disease has spread, particularly among those who visited Mexico City.
"Here in Cancun, up to a dozen New York students who travelled to the resort for spring break have developed what is being described as a mild case of the flu.
"While that's being treated, it is a concern for authorities given the number of tourists who flock to Cancun - particularly during the spring break."
After Mexico, the United States has so far recorded the highest number of cases, with at least 40 confirmed, non-fatal, swine flu patients in five states.
Some 47 people have been quarantined and are being tested in Australia. Authorities are searching for another 300 who could have come into contact with the virus, reports news.com.au.
In Europe, the first cases of swine flu were confirmed in Britain and Spain as travellers returned from trips to Mexico.
There have been six confirmed cases in Canada and further suspected infections have been reported in New Zealand, Colombia, France and Israel.
And several suspected cases of swine flu have been reported in China, according to the WHO.
The spread prompted the World Health Organisation to raise its alert level, but it stopped short of declaring a global emergency.
"At this time, containment is not a feasible option," said WHO Assistant Director-General Keiji Fukuda.
WHO officials warned that the new strain of flu, apparently created when human and avian flu viruses infected pigs and became mixed, could further mutate.
It could take 4-6 months before the first batch of vaccines are available to fight the virus, they added.
Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus, while US president Barack Obama said the outbreak was reason for concern, but not yet "a cause for alarm".
Many of the cases outside Mexico have been relatively mild, with symptoms including fever, coughing, joint aches, severe headache and in some cases, vomiting and diarrhoea.
The outbreak has prompted fears that the world may finally be experiencing the flu pandemic that health officials have been warning about for years.