Experts suspect the current spread of bird flu in Asia, Africa and Europe is mainly a result of trade in infected live birds rather than transmission through wild birds, the U.N. official coordinating the global fight against avian influenza said Friday.

Dr. David Nabarro said investigators looking into the cause of a bird flu outbreak at a commercial turkey farm in Britain are now focusing on a possible link with the transfer of partly processed birds from a farm in southeastern Hungary, where there was an outbreak last month.

Britain's Department of Environment, Food and Rural Affairs said preliminary inquiries indicated the strain of H5N1 bird flu found at the British farm was identical to the strain found last month in Hungary. Environment Minister Ben Bradshaw said the government was investigating whether there were "bio-security breaches" at the British farm, owned by Bernard Matthews PLC, Europe's biggest turkey producer.

If there was movement of poultry -- either live or dead -- from an area where H5N1 bird flu had been found, Nabarro said, "that will be an action which goes against the guidance of the United Nations system and also the guidance of other authorities who have jurisdiction over what goes on."

The U.N. bird flu chief said the recent upsurge in H5N1 bird flu outbreaks around the world is not a surprise.

"Since 2003, we've seen a rise in the number of reported outbreaks in poultry and indeed of human cases ... between the period December-April, and we expect that there will be more outbreaks," Nabarro said, adding that new cases could emerge through June.

During the last two months, he said, there have been new outbreaks of H5N1 avian influenza virus in Indonesia, Vietnam, South Korea, China, Thailand, Japan, Egypt, Hungary, Nigeria and Britain -- and a new outbreak was reported in Turkey.

"And human infections have been confirmed in China, Egypt, Nigeria and Indonesia and they're suspected in other locations as well," he said.

H5N1 has prompted the slaughter of millions of birds across Asia since late 2003 and caused the deaths of more than 160 people worldwide, around a third of them in Indonesia, according to the World Health Organization.

Most people killed so far have been infected by domestic fowl and the virus remains very hard for humans to catch. Nabarro said about half the people infected die.

But experts fear it could mutate into a form that easily spreads among humans, sparking a pandemic with the potential to kill millions.

Nabarro said governments and people around the world must maintain their focus on avian influence "because there is still the possibility ... of a human pandemic."

In autumn 2005, he said, bird flu experts started to see large numbers of migratory birds dying in northern China and southern Russia and there was serious concern that wild birds were becoming a significant factor in transmitting the H5N1 virus into commercial poultry populations.

"During this northern winter season, we have not had the same kind of wild bird die-offs," Nabarro said.

"We're not anticipating the same role for wild birds that we had a year ago, and we suspect that one of the reasons for the current spread has more to do with trade in live birds than to do with the movement of the virus through wild birds," he said.

"That said, it's still a bit difficult to explain some of the outbreaks that have occurred, for example, in Japan and in the Republic of Korea," Nabarro said.

He said countries with widespread outbreaks of bird flu are struggling to control the spread, including Indonesia, Egypt and Nigeria.