World health officials have issued their strongest warning yet about the global threat posed by bird flu.
The virus is usually passed to humans from infected birds
The world is now in the gravest possible danger of a human flu pandemic triggered by the virus, Dr Shigeru Omi told a conference in Vietnam.
The World Health Organisation fears bird flu may get deadlier if it mutates into a form that could be easily transmitted between humans.
Thirteen people have died in Vietnam from the virus since December.
Governments need to take quick action to control the virus's spread, Dr Omi added.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
Bird flu is proving resistant to treatments and has appeared in animals including cats and tigers which had not been considered susceptible to the virus.
Last year bird flu swept through poultry in a dozen countries and killed 45 people.
About 140 million birds died or were slaughtered in an effort to contain the outbreak.
"If the virus becomes highly contagious among humans, the health impact in terms of deaths and sickness will be enormous," Dr Omi said.
Dr Omi told the United Nation's Food and Agriculture Organization conference in Ho Chi Minh City "the world is now in the gravest possible danger of a pandemic".
Such outbreaks usually occur every 20 to 30 years, he said.
The last one was nearly 40 years ago, so by this measure an outbreak was overdue.
Duty to help
Dr Omi said avian flu had the potential to kill more people than Severe Acute Respiratory Syndrome (SARS), which killed about 800 people two years ago.
He said WHO was calling on governments to develop contingency plans in case of an outbreak.
However, he added that the virus, also called H5N1, had not yet "gained the potential for efficient human-to-human transmission" which would make it far more deadly.
The conference is focusing on controlling the spread of the disease amongst poultry populations in South Asian countries.
Curbing the disease in this way would also help prevent bird flu spreading to countries outside the area, delegates said.
Dr Samuel Jutzi, of the Food and Agriculture Organization, told the conference: "There is an increasing risk of avian influenza spread that no poultry-keeping country can afford to ignore."
The WHO has previously called on governments around the world to stockpile vaccines so they would be prepared should the feared pandemic occur.
The United States has contracted for 4 million vaccine doses, while Italy and France anticipate stockpiling 2 million doses each, New Scientist magazine recently reported.
In the UK, the Department of Health has said it has no plans to stockpile vaccines, because of the difficulty of designing one in advance which will act against a mutated virus.
However, it has said it will provide anti-viral drugs to key groups such as health workers and emergency services.
Delegates at the three-day Vietnam conference heard avian flu was now entrenched in the bird population of south-east Asia and would probably persist for many years.
UN officials said it was the common duty of governments around the world to provide practical support to South East Asia to help tackle the disease.