United Nations officials say the bird flu virus that has killed more than 100 people and millions of fowl in various parts of the world continues to pose a major threat, despite successes by some countries in combating it. But they say the recent outbreak in Burma is more serious than previously thought, causing new concerns.

The United Nations coordinator on avian influenza, David Nabarro, says although international efforts to combat bird flu have been largely successful to date, the world community cannot become complacent.

David Nabarro

"Great progress in some places, still a lot to do in others," he said, " but overall, don't let any of us think that the problem has somehow gone away. It's not. It's there. And the current situation we're in of control in some countries is good, but it's fragile."

Nabarro made the remarks in Bangkok during a five-nation trip to Asia, where the virus reappeared more than two years ago. Since then, the disease has killed 109 people in nine nations and has spread to Europe, Africa and the Middle East. It has also led to the culling of millions of fowl.

Nabarro praised the response by Thailand and Vietnam, two of the countries hit hardest by the bird flu, and he noted a strengthening commitment by the Chinese government to combat the disease. But he added that greater efforts are needed to fight the virus in nations such as Indonesia and Cambodia, where new outbreaks continue.

Another expert, the regional representative for the U.N. Food and Agriculture Organization, He Changchui, said Burma, also known as Myanmar, is a new and significant concern, in part because of public ignorance of the disease. He said, although bird flu only was reported there last month, it has spread to 100 districts, mainly in the central and northern parts of the country.

He Changchui

"The issue there (in Burma) is that the awareness is rather poor. The information is not that comprehensive," he said.

He said that Burma, one of Asia's poorest countries, does not have the personnel or facilities to deal with the outbreak but added that U.N. teams will focus on it in the coming weeks.

The military government in Burma has culled half a million birds and has requested U.N. assistance. It has also begun a public awareness campaign. But tight government controls on the news media and foreign aid groups has limited the development of systems that could be used to combat the disease.

Officials note that so far bird flu almost exclusively has infected humans who come into close contact with sick birds.

However, they worry that if the mutating virus gains the ability to spread from human to human, it could cause a global pandemic that could kill millions of people.

Nabarro says that the spread of the virus to Africa and Europe increases this possibility.

"Therefore increases the potential for that virus to lead to sporadic infection in humans. It also increases the chance of a very dangerous mutation occurring," he said.

The U.N. experts said that unless a human vaccine against bird flu is developed, the best way to avoid a pandemic is through surveillance and prevention. In other words, outbreaks at farms and live poultry markets must be found and isolated quickly.