A rare outbreak of human swine flu has killed at least 60 people in Mexico and spread to the United States where authorities are on alert, the World Health Organisation said on Friday.
The WHO has identified swine influenza as a potential source of a human flu pandemic. Fadela Chaib, a spokeswoman for the UN health agency, said hundreds of cases of suspected swine flu -- which regularly hits pigs but rarely affects humans -- have been reported in Mexico.
"To date there have been some 800 suspected cases with flu-like illness, with 57 deaths in the Mexico City area," Chaib added.
Twenty four suspected cases and three deaths were also recorded in San Luis Potosi in central Mexico.
Chaib said there are now seven known cases in the southwestern United States in three clusters -- five in California and two in Texas.
"Meanwhile in Mexico unusual end of season influenza activity began to be noticed at the end of March, peaking in April," the spokeswoman told journalists.
"The virus is being described in the United States as a new subtype of A/H1N1. Mexico so far hasn't said anything about the virus, what type of virus it is," said Chaib.
Most of the Mexican cases were found in healthy young adults with no known record of prior illness.
The WHO said it was in constant contact with health authorities in the United States and in Mexico.
Human outbreaks of H1N1 swine influenza virus were recorded in the United States in 1976 and 1988, when two deaths were recorded, and in 1986. In 1988 a pregnant woman died after contact with sick pigs according to the WHO.
In recent years the global focus for a pandemic has shifted to the H5N1 bird flu virus, which has spread from poultry to humans, especially in Asia.