Ukraine says it has detected the strain of bird flu potentially fatal to humans in several villages in the Crimean peninsula.A massive cull is underway in the area where some 2,500 birds reportedly died from the virus within hours. Authorities have sealed off the area and rounded up tens of thousands of domestic poultry from local farms. Bird flu has spread from Asia to several European countries – its pathogenic strain H5N1 has been detected in four countries, including Russia, Ukraine, Romania and Turkey. H5N1 has killed some 70 people in Asia since 2003. Experts fear the virus could mutate into a form which could easily be transmitted from human to human, leading to a pandemic which could kill millions of people. It is unclear how the virus is currently spread. There are conflicting opinions on whether it stems from poultry imports or from migratory birds. Residents in the affected villages in Ukraine claim signs of the illness had already been detected in September but no action was taken.