Mexico City  - The death toll from a spreading wave of influenza in Mexico has climbed to 81, Health Minister Jose Angel Cordova Villalobos said Saturday evening.

Twenty of the lethal infections have been definitively linked to the newly emerging strain of swine flu, he said. All told, 1,324 people have been admitted to hospital for examination.

In the United States, 11 non-lethal cases of a similar strain of influenza have been identified by the Centres for Disease Control, with tests pending on another 10 cases.

The Geneva-based World Health Organization declared the outbreak of swine flu in Mexico and the United States a "public health emergency of international concern."

Cordova Villalobos said that schools would remain closed next week in Mexico City, where the flu has hit hardest.

The health minister dismissed speculation in the Mexican media about US President Barack Obama's possible exposure to the virus when he was in Mexico City ten days ago, just after the flu started breaking out there.

Reforma newspaper had reported that Obama had had contact at a museum with a reknown archeologist, Felipe Solis Olguin, who died of a combination of diabetes and pneumonia shortly afterwards.

Cordova Villalobos clarified the man's cause of death, saying the pneumonia was not related to influenza, but rather to a combination of an unnamed pre-existing condition and a sudden onset of pneumonia that was not related to influenza.

In Washington, the White House reassured CNN that the president was not suffering from any flu symptoms.

In Britain, the Sunday Times reported that a British Airways cabin crew member had been taken to a hospital with influenza symptoms after becoming sick on a flight from Mexico City into Heathrow Airport.

Growing concern by health officials focussed on the apparent human-to-human transmission of the swine flu, which normally is only transmitted from animal to human. (dpa)