The World Health Organization warned that as the pandemic death toll rose, production of swine flu vaccines will fall 'substantially' short of the amount needed to protect the global population.
"Current supplies of pandemic vaccine are inadequate for a world population in which virtually everyone is susceptible to infection by a new and readily contagious virus," WHO director general Margaret Chan said in a statement.
Despite new evidence that only one dose of the vaccines currently being tested will be enough for most people, WHO spokesman Gregory Hartl said output next year will be "substantially less" than the 4.9 billion doses annual production forecast.
Some 25 pharmaceutical laboratories working on vaccines have indicated that weekly production is lower than 94 million doses, he said.
In May, the WHO had forecast a weekly output of 94.3 million doses if full scale vaccine production was launched.
But pharmaceutical companies have in recent weeks slashed their production expectations due to poorer than expected yields from the so-called "seed virus" strains developed by WHO-approved laboratories.
Amid growing fears that poorer nations will not get enough vaccines, the United States led nine countries which on Thursday pledged to make 10 percent of their swine flu vaccine supply available to other nations in need.
The UN health agency's chief applauded the move by the United States, Australia, Brazil, Britain, France, Italy, New Zealand, Norway and Switzerland.
"Given that current demand outstrips supply, these donations, together with the doses pledged by manufacturers, will help increase supplies of pandemic vaccines to populations that would otherwise not have access," said Chan.
Swine flu cases are expected to increase as the Northern Hemisphere enters its winter season. Britain has already reported a new surge in caseload.
The WHO also announced Friday that the global flu death toll has reached 3,486, up 281 from a week ago.
The UN agency said the Americas region still has the highest death toll, at 2,625. The Asia-Pacific reported 620 fatalities, while Europe recorded at least 140 deaths. In the Middle East, 61 people succumbed to the virus while in Africa, 40 people have died from it.
The WHO also said flu activity was "above the seasonal baseline" in the United States and that it has reached epidemic levels in France and Japan.
Transmission is rife in central and south America and Asia, it added, while in temperate regions of the Southern Hemisphere, such as Australia and South Africa, flu activity is declining.
Experts have previously predicted that about one third of the world's population of more than 6.5 billion people could be affected by A(H1N1). But they stress that so far most victims are suffering only mild symptoms.