The deadly H5N1 strain of bird flu is likely to spread to more and more countries, a World Health Organization official warned on Monday.

The strain, which has killed over 60 people in southeast Asia, appears to have travelled extensively in the latter half of 2005. It has affected birds in China, Mongolia, Russia, Kazakhstan and most recently reached Europe.

It is now believed that the virus has been carried and spread by migratory birds, which may mean other continents are at risk.