* Mexican government announces fatal swine flu outbreak
* New mixture of viruses never seen before
* Some Mexico City residents don surgical masks (Updates with color, quotes)
By Noel Randewich and Armando Tovar
MEXICO CITY, April 24 (Reuters) - A deadly strain of swine flu never seen before has broken out in Mexico, killing at least 16 people and raising fears it is spreading across North America.
The World Health Organization 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 United States.
Mexico canceled classes for millions of children in its sprawling capital city and surrounding areas on Friday 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 told the Televisa network.
He linked the disease in Mexico to a new kind of swine flu that struck seven people in California and Texas.
The U.S. Centers for Disease Control and Prevention said the virus in the United States was a never-before-seen mixture of viruses typical among pigs, birds and humans. All seven American patients have recovered.
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.
Mexico City, one of the world’s biggest cities and home to some 20 million people, was quieter than usual on Friday morning. Normally choking traffic was less chaotic in the absence of school buses and parents driving kids to school.
Many people waiting to enter subway stations had their faces covered with surgical masks.
The virus is an influenza A virus, carrying the designation H1N1. It contains DNA typical to avian, swine and human viruses, including elements from European and Asian swine viruses, the CDC has said.
WHO said about 60 people in Mexico have died from the disease. The Geneva-based U.N. agency said it was in daily contact with U.S., Canadian and Mexican authorities and had activated its Strategic Health Operations Center (SHOC) — its command and control center for acute public health events.
Surveillance for and scrutiny of influenza has been stepped up since 2003, when H5N1 bird flu reappeared in Asia. Experts fear this strain, or another strain, could spark a pandemic that could kill millions. (Additional reporting by Maggie Fox in Washington and Stephanie Nebehay in Geneva; Writing by Frances Kerry, Editing by Eric Walsh)