The swine flu virus hasn't mutated into a more deadly strain but there are early signs it is developing resistance to vaccine, the World Heath Organization's chief said Monday.

Authorities are monitoring closely whether the virus was morphing into more virulent forms that would make it deadlier, the organization's Director-General Margaret Chan said.

"We are not seeing that situation right now," Chan told reporters as the WHO convened a conference in Hong Kong.

The WHO says the swine flu virus — also known as H1N1 — has killed almost 3,486 people worldwide as of Aug. 13. South America and North America account for the majority of deaths.

For now, the infection is generally mild and most people recover without treatment. But should it become deadlier, developing nations could be especially vulnerable because those populations lack adequate health care and are already fighting a myriad of diseases including AIDS, malaria and tuberculosis.

Chan said manufacturers were on track to develop billions of new doses of the vaccine over the next year. The vaccine is highly effective against the swine flu virus, though there were a small number of instances — about 25 in the world — of a vaccine-resistant flu.