While the overall situation is improving, that doesn't mean it is under control globally, Dr David Nabarro said.

"We have the situation in countries where it is still quite entrenched. But it does mean that in the rest of the world, there is a great deal of vigilance and action under way," he said in a briefing for reporters.

"We remain very concerned about Indonesia, where the disease seems to be concentrated among poultry, particularly in Western Java.

Dr Nabarro attributed the general improvement to massive spending on raising poultry, and praised South Korea and Britain for helping bring outbreaks under control and the United States and Australia for their preparedness. He said about 160 nations in all have prepared some kind of plan to deal with bird flu, up from 144 less than a year ago.

He also cited as a factor French drug company Sanofi Pasteur's pledge this week to donate over three years 60 million vaccines against the hard-to-treat H5N1 flu strain. That comes on top of British drug company GlaxoSmithKline's pledge last year to donate 50 million doses of H5N1 vaccine to help the World Health Organization set up a global stockpile.

"It's prudent to continue to be prepared, especially as genetic studies of the current bird flu virus show that it is continuously evolving, even though it hasn't become capable of sustained transmission among humans," he said.

Dr Nabarro said that bird flu in poultry and wild birds had spread to 60 nations by last year, up from nine nations only several years earlier. The increase is thought to have resulted more from trade in infected live birds than by transmission through wild birds, whose migrations change with weather.

Around the world, the upsurge in H5N1 bird flu outbreaks has led to the slaughter of millions of birds across Asia since late 2003. Sporadic human-to-human transmission of the H5N1 strain has been reported in Hong Kong, Viet Nam and Indonesia, but none of the cases has been proven, and officials determined there was no epidemiological significance because the spread was not sustained.

Most people killed so far have been infected by domestic fowl, and the virus remains difficult for humans to catch. About half the people infected die. Experts fear, though, that it could mutate into a form that easily spreads among humans, sparking a pandemic that some have estimated could kill anywhere from 5 million to 150 million.

More than 40 million people died in the 1918 flu pandemic, the worst in history. The 1957 and 1968 pandemics had lower death rates.