GENEVA (AFP) - The World Health Organisation was on high alert Saturday in the face of an outbreak of a new multi-strain swine flu virus that has killed up to 60 people in Mexico and also infected the United States. The outbreak of the flu virus transmitted from human to human is a serious situation with a pandemic potential, the head of the World Health Organisation said Saturday. A new virus is responsible for the cases reported in Mexico and the United States, WHO Director-General Margaret Chan said in a telephone Press conference, adding: It is a serious situation which needs to be closely followed. How the situation will evolve is unpredictable, she said, urging other countries to increase vigilance. This virus has clearly a pandemic potential, she added. In Mexico the main outbreak has hit the capital, Mexico City, with between 18 and 20 confirmed deaths due to the virus, but San Luis Potosi in the centre, with three dead, and Mexicali on the US border, have also suffered cases of the disease. The virus detected in 12 fatal Mexican cases is also genetically identical to that which affected eight people in the US states of California and Texas, though all of those recovered. In addition 75 students in New York showing flu-like symptoms have been undergoing tests, according to CNN. Authorities in Mexico, where 40 more deaths are suspected to have resulted from the disease, and some 1,000 patients are under observation, are already using the term epidemic, but the WHO has not yet gone so far. A pandemic occurs when there is a new virus to which few people have resistance, the virus is easily transmissible and sustainable within a population, and causes severe illness.