"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," said the WHO in a statement.
"The pandemic will persist in the coming months as the virus continues to move through susceptible populations," it added.
Nigel Dimmock, a professor at the University of Warwick's biological sciences department, said it was "surprising" that the WHO has declared the virus a dominant strain "so soon." The development could have implications on vaccines, he said. "If the pandemic H1N1 has replaced the other strains, as happened in 1957 and 1968, then you don't need the old vaccine," he said. "But if they continue to co-circulate, even if it is dominant, then you are going to need a vaccine with three type-A components, and a seasonal B."
Around the world, 2,185 people have died from the virus since it was uncovered in April. Some tropical countries are already reporting "moderate strains" on their healthcare systems amid surges in infections.
"Many countries in tropical regions, represented by central America and tropical regions of Asia, continue to see increasing or sustained high levels of influenza activity with some countries reporting moderate strains on the healthcare system," said the UN health agency.
In Japan, the flu has become an epidemic, signaling that the annual flu season could be a long one.
Swine flu also showed several significant differences from seasonal flu, the WHO pointed out.
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 reported, WHO said, adding that highly specialized 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 hospitalized cases required intensive care.
"Preparedness measures need to anticipate this increased demand on intensive care units, which could be overwhelmed by a sudden surge in the number of severe cases," it said.
The UN health agency reiterated that pregnant women and those with medical conditions such as asthma, cardiovascular disease and diabetes count among the most vulnerable to the flu.
In addition, studies are indicating that minority groups and indigenous populations have up five times higher risk of developing severe illness or dying from the disease than the general population.
There appears to be just one bright spot, HIV-infected people have not shown increased risk of developing severe or fatal illness when they catch swine flu.
This could be "reassuring news" for countries where HIV is prevalent, said the WHO.