MEXICO CITY (Reuters) - A deadly strain of swine flu never seen before has broken out in Mexico, killing as many as 60 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. It said about 60 people had died in Mexico. Mexico's government said it had confirmed that at least 16 people had died of the swine flu in central Mexico and that there could be another 45 fatal victims.