The World Health Organisation chief says a new flu virus suspected of killing up to 81 people in Mexico has the potential to become a pandemic.
It's stopped short of declaring the flu outbreaks in Mexico and the United States a pandemic, but WHO director-general Margaret Chan said the outbreak was "a health emergency of international concern" and must be closely monitored.
Dr Chan urged all countries to boost their surveillance for any unusual outbreaks of influenza-like illness and severe pneumonia.
"It has pandemic potential because it is infecting people," Dr Chan said in Geneva.
"However, we cannot say on the basis of currently available laboratory, epidemiological and clinical evidence whether or not it will indeed cause a pandemic" she said.
The organisation's emergency committee of experts agreed that more information was needed before making a decision on a possible change to the WHO's pandemic alert phase, currently 3 on a scale of 1 to 6.
How swine flu spreads in humans
The US Centers for Disease Control and Prevention (CDC) says most cases of swine flu in humans occur when people come into direct contact with infected pigs.
Pigs can catch both human and avian or bird flu, and when flu viruses from different species infect pigs, they can mix inside the pig and new viruses can emerge, the CDC says.
Pigs can pass mutated viruses back to humans and they can be passed from human to human. Transmission among humans is thought to occur in the same way as seasonal flu - such as through coughing or sneezing.
People cannot catch swine flu from eating pork or pork products. Cooking pork to an internal temperature of 71 degrees Celsius kills the swine flu virus as it does other bacteria and viruses.
Phase 3 on the scale indicates a new influenza virus subtype is causing disease in people, but is not yet spreading efficiently and sustainably among people, the organisation's website says.
The last flu pandemic was in 1968 when "Hong Kong" flu killed about a million people globally.