The U.N. health agency (search) said Friday it was impossible to estimate how many people might die from a new influenza pandemic (search), adding that it has warned countries to prepare for a death toll of up to 7.4 million.

"We think that this is the most reasoned position," said World Health Organization (search) spokesman Dick Thompson, warning that "you could pick almost any number."

On Thursday, Dr. David Nabarro -- the new U.N. coordinator for avian and human influenza -- warned that the "range of deaths could be anything between 5 and 150 million" from a new pandemic.

Several scientists have made predictions on how many people could die in a flu pandemic, and estimates have ranged from less than 2 million to more than 100 million.

The number of deaths will depend largely on how contagious and lethal the virus is -- two factors that cannot be known until the pandemic strain emerges.

However, even though several estimates could be plausible, WHO "can't be dragged into further scaremongering," Thompson told reporters.

"One of those numbers will turn out to be right," Thompson said. "We're not going to know how lethal the next pandemic is going to be until the pandemic begins."

The H5N1 strain of bird flu has swept through poultry populations in large swathes of Asia since 2003, jumping to humans and killing at least 65 people -- more than 40 of them in Vietnam -- and resulting in the deaths of tens of millions of birds.

Most human cases have been linked to contact with sick birds. But WHO has warned that the virus could mutate into a form that spreads easily among humans -- possibly triggering a global pandemic that could kill millions.

Southeast Asia's agriculture ministers endorsed a regional plan Friday to combat bird flu and pledged to cooperate with international agencies in a move they hope will win enough aid to halt the disease before it becomes a catastrophic epidemic.

The ministers from the 10-member Association of Southeast Asian Nations, meeting in Tagaytay, Philippines, said in a statement that the flu requires "an all-out coordinated regional effort."

The meeting in the Philippines ended with the ASEAN ministers' joint statement endorsing a regional plan for control and eradication of bird flu over three years from 2006 and directing a new task force to urgently formulate "a detailed action plan for implementation and proceed to identify potential sources of funding."

The plan covers eight strategic areas, including a disease surveillance and alert system, vaccination, improving diagnostic capability and establishing disease-free zones.

The regional framework dovetails with a three-year plan drafted by the U.N. Food and Agriculture Organization, the World Organization for Animal Health, or OIE, and the WHO in May, to be presented to international donors in December for implementation next year.

"What we hope to do at the (regional) task force level is to supplement what is going on, what is being done by each individual country and to work with FAO, OIE and WHO," Singapore's Minister for National Development Mah Bow Tan told a news conference.

ASEAN Deputy Secretary General Wilfrido Villacorta said given the gravity of the problem "we are confident that we shall continue to have the support of our dialogue partners."

The ASEAN animal health trust fund formally established at the meeting "gives the signal to potential donors that ASEAN member countries are serious about eradicating the avian flu as well as other diseases that are facing the region," he added. Pledges of $2 million have been made for the fund, which is separate from a regional one for bird flu that ASEAN hopes to have, officials said.

ASEAN comprises Malaysia, Singapore, Indonesia, the Philippines, Thailand, Vietnam, Brunei, Cambodia, Laos and Myanmar.

"It's important that we have the political commitment of the region so we can effectively invite the donors to back the program (on bird flu)," said Subhash Mozaria, FAO chief technical adviser.