WASHINGTON (Reuters) - A new strain of H5N1 bird flu has shown up among birds in Africa in a worrying development, the United Nations Food and Agriculture Organization reported on Monday.

The new strain of H5N1 avian influenza is genetically different from the strains that circulated in Nigeria during earlier outbreaks in 2006 and 2007 and is new to Africa, the FAO said.

"It is more similar to strains previously identified in Europe (Italy), Asia (Afghanistan) and the Middle East (Iran) in 2007," the FAO said in a statement.

"The detection of a new avian influenza virus strain in Africa raises serious concerns as it remains unknown how this strain has been introduced to the continent," said Scott Newman, International Wildlife Coordinator of FAO´s Animal Health Service.

"It seems to be unlikely that wild birds have carried the strain to Africa, since the last migration of wild birds from Europe and Central Asia to Africa occurred in September 2007 and this year´s southerly migration into Africa has not really started yet," Newman added.

"It could well be that there are other channels for virus introduction: international trade, for example, or illegal and unreported movement of poultry. This increases the risk of avian influenza spread to other countries in Western Africa."

Avian influenza is common, but the H5N1 strain is particularly worrying both to poultry producers and doctors. The highly pathogenic H5N1 strain has swept through flocks in many parts of Asia, Europe and Africa.

It rarely infects people but has killed 243 out of 385 known to have been infected since 2003, according to the World Health Organization. It has killed or forced the slaughter of 300 million birds.

The fear is it could mutate into a form that people can easily catch and transmit; this could cause a pandemic that could kill hundreds of millions of people.

And H5N1 has become entrenched in a way that no other form of bird flu has, WHO says.

"Uncertainty about virus spread and transmission is a major challenge for control campaigns. Increased surveillance is key to monitor the situation and keep track of virus spread," said FAO chief veterinary officer Joseph Domenech.

Just last month Nigeria reported its first outbreak of H5N1 in nearly 10 months, among chickens and ducks in the northern cities of Kano and Katsina.

(Reporting by Maggie Fox, editing by Will Dunham and Todd Eastham)