Health authorities commonly fog mosquito-infested areas with clouds of insecticide, and resistance has long been a concern, but the scale of the problem was not well understood.
Japanese scientist Shinji Kasai and his team examined mosquitos from several countries in Asia as well as Ghana and found a series of mutations had made some virtually impervious to popular pyrethroid-based chemicals like permethrin.
"In Cambodia, more than 90 per cent of Aedes aegypti mosquitoes have the combination of mutations that results in an extremely high level of resistance," Kasai told AFP.
He found some mosquito strains had 1,000-fold resistance, compared to the 100-fold seen previously.
That meant insecticide levels that would normally kill almost 100 per cent of mosquitoes in a sample killed only around seven per cent of the insects.
Even a dose 10 times stronger killed just 30 per cent of the super-resistant mosquitoes.
"The resistance level that we found in mosquitos in Cambodia and Vietnam is totally different," said Kasai, director of the Department of Medical Entomology at Japan's National Institute of Infectious Diseases.