The head of the World Health Organisation said the swine flu outbreak in Mexico and the United States could develop into a pandemic.
WHO Director-General Margaret Chan says the outbreak involves “an animal strain of the H1N1 virus, and it has pandemic potential”.
Ms Chan says it is too early to say whether a pandemic will actually occur.
The global health body has advised countries around the world to look out for similar outbreaks following the discovery of related strains on both sides of the Mexico-US border.
At least 62 people in Mexico have died from pneumonia after contracting a flu-like virus.
WHO says some tested positive for a strain that sickened at least seven in the south-western US.
No deaths have been reported in the US.