World Health Organisation director general Margaret Chan said the pandemic would "test the world on the issue of fairness," as she delivered an address to a meeting of Southeast Asian health ministers in Kathmandu.
"I believe it will reveal in a measurable and tragic way the consequences of decades of failure to invest adequately in basic health systems and infrastructure," said Chan.
"It will show what the failure to care about equity in international policy really means in life and death terms."
At least 2,837 people around the world have died from the swine flu virus since it emerged in April, according to the latest WHO figures.
Southeast Asia, home to many of the world's poorest people, has so far been hit relatively lightly.
But Chan warned against complacency in the WHO's 11 Southeast Asian member countries -- Bangladesh, Bhutan, North Korea, India, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand and East Timor.
"I can assure you, the pandemic will come to all your countries, and this is not the same as seasonal influenza," Chan told the gathered ministers, adding the pandemic would be "with us for quite some months to come."
The WHO said last month the virus had become the most prevalent flu strain.
Some tropical countries were already reporting "moderate strains" on their healthcare systems amid surges in infections, the UN health agency said.