A deadly strain of swine flu has broken out in Mexico, killing at least 16 people and raising fears it is spreading across North America.

The World Health Organisation said it was concerned about what it called 800 influenza-like cases in Mexico, and also about a confirmed outbreak of a new strain of swine flu in the US.

Mexico has cancelled classes for millions of children in the capital city and surrounding areas after authorities noticed a higher number of deaths involving flu-like illness than normal in recent weeks.

'It is a virus that mutated from pigs and then at some point was transmitted to humans,' Health Minister Jose Angel Cordova said.

He linked the disease in Mexico to a new kind of swine flu that affected seven people in California and Texas.

The Mexican government warned people not to shake hands or kiss when greeting or share food, glasses or cutlery for fear of contracting the flu.

The US Centers for Disease Control and Prevention said the virus in the US was a never-before-seen mixture of viruses typical among pigs, birds and humans.

All seven American patients have recovered. The White House said President Obama had been informed of the outbreak.

The Geneva-based WHO agency said it was in daily contact with US, Canadian and Mexican authorities.