More than 80 people have died from flu-related pneumonia in Mexico - where the virus originated.
Over 1,300 people are now thought to have contracted the virulent H1N1 swine influenza after it mutated into a form that spreads from human to human.
The disease has already spread to the United States, where 11 cases have been confirmed in Texas, California and Kansas.
A group of school children from New York is also undergoing tests, where a further eight are suspected to have contracted the virus, health officials said.
Another group of 10 school children in New Zealand have tested positive for influenza after returning from Mexico, which the health minister there has said is "likely" to be the H1N1 strain.
A 26-year-old Israeli man has also been admitted to hospital after returning from a trip to Mexico with flu-like symptoms.
The UK is on alert after the global health watchdog warned countries to look out for unusual flu cases, especially among passengers returning from affected areas.
Yesterday a British Airways cabin crew member was taken to hospital with flu-like symptoms after returning from Mexico City.
But a spokesman for Northwick Park Hospital in London said tests confirmed he did not have the deadly strain.
World Health Organisation director-general Margaret Chan said the outbreak constituted a "public health emergency of international concern".
It means nations will be expected to step up reporting and surveillance of the contagious respiratory disease, which she said had "pandemic potential".
Mexico's health minister Jose Angel Cordova said all schools have been shut in Mexico City, the surrounding area and the central state of San Luis Potosi until May 6.
The UK Health Protection Agency called the outbreak unusual and called for further investigation and vigilance among all countries involved.
The Agency said those with flu-like symptoms who have recently returned from the affected areas should stay at home to limit contact with others and seek medical advice from a local health professional or NHS Direct.
But an HPA spokesman added: "No cases of swine flu have been identified in the UK or anywhere in Europe."
"The HPA and the NHS have systems in place, which will alert public health authorities of any unusual strain circulating in the UK."
Britons are not currently being advised to avoid travelling to affected areas of Mexico and the US.
The H1N1 strain of swine flu is usually only seen in pigs - but in humans can cause serious symptoms including fever and fatigue.