Swine flu spreads four times faster than other viruses and 40 percent of the fatalities are young adults in good health, the world's top health official warned in an interview.
"This virus travels at an unbelievable, almost unheard of speed," World Health Organisation Director General Margaret Chan told France's Le Monde daily in an interview.
"In six weeks it travels the same distance that other viruses take six months to cover," Chan said.
"This is the most worrying fact," she said, adding that "up to 30 percent of people in densely populated countries risked getting infected."
Chan's warning came a day after the WHO said the virus had overtaken others to become the most prevalent flu strain.
"Evidence from multiple outbreak sites demonstrates that the A(H1N1) pandemic virus has rapidly established itself and is now the dominant influenza strain in most parts of the world," the UN agency said in a statement.
"The pandemic will persist in the coming months as the virus continues to move through susceptible populations," it added.
Chan underlined that emergency and healthcare services in several countries had come under strain and stressed that resources allocated for cancer patients and those suffering from heart disease should not be diverted.
"One must not rob Peter to pay Paul," she said. "All governments must prepare for the worst."
She said the most important thing in the battle against the virus was "political leadership."
More than 2,180 people around the world have died from the virus since it emerged in April, according to the latest WHO figures.
Chan also said that it could be months before sufficient vaccine is available to combat the pandemic.
She put world production capacity at 900 million doses a year, for a global population of 6.8 billion people.
Even if this was an unprecedented effort, and authorities were speeding up procedures for getting vaccines to the market, there should be no question of compromises on their safety and effectiveness, Chan said.
Britain and France received their first batches of swine flu vaccine this week. Australia on Friday said a massive swine flu vaccination programme would start in October and Turkey hopes the first supplies of the vaccine will come by that time.
While 90 percent of severe and fatal cases occur in people aged above 65 in seasonal flu, most of those who die from swine flu are under the age of 50.
A "very severe form of disease" affecting the lungs and causing severe respiratory failure among young and healthy people was being reported, WHO said Friday, adding that highly specialised care was required.
Large numbers of such patients could therefore "overwhelm" intensive care units and disrupt the provision of care for other diseases, it warned.
In the southern hemisphere where the flu-prone winter season is tailing off, the WHO said cities in several countries had reported that nearly 15 percent of hospitalised cases required intensive care.