Dozens of people have died and hundreds of others have been infected in a viral outbreak in Mexico suspected to have been caused by a strain of swine flu.
The WHO says it believes the virus is behind 60 deaths, mostly in and around the capital since mid-March.
Mexican authorities have closed schools in affected areas and a vaccination campaign is being launched.
Seven non-fatal cases of a new form of swine flu have also been confirmed in the southern United States.
Experts will carry out tests to determine if the two viruses are linked.
‘Mutated from pigs’
WHO spokeswoman Fadela Chaib said “unusual end of season influenza activity” was noticed in Mexico starting from the end of March.
Fifty-seven people had died in Mexico City from flu-like symptoms, she said, and another three in San Luis Potosi in central Mexico. There are around 800 suspected cases, she said.
The Mexican government said swine flu had been confirmed in at least 16 deaths, and there were dozens of other suspected deaths.
Health Minister Jose Angel Cordova said the virus “mutated from pigs and then at some point was transmitted to humans”.
It is not yet clear whether it is the same virus that left seven people sick in the US states of Texas and California.
US experts say they were suffering from a new form of swine flu that combined pig, bird and human viruses.
“This is the first time that we’ve seen an avian strain, two swine strains and a human strain,” Dave Daigle, a spokesman for the US Center for Disease Control and Prevention (CDC), told AFP.
The CDC said none of the seven victims had been in contact with pigs, which is how people usually catch swine flu.
It was tracking those who had been in contact with the seven to see if the were ill, it said. All seven had made a full recovery.