The fight against malaria could be undermined by the emergence on the Thai-Cambodian border of strains that are resistant to the most potent type of drug, the World Health Organisation said on Wednesday.

Artemisinin, a compound extracted from a Chinese herb, is regarded by medical experts as the best drug against malaria and is recommended for use in combination with other medicines to stop the development of resistance.

But the WHO said there was growing evidence that parasites resistant to artemisinin had emerged along the border between Cambodia and Thailand, where workers walk for miles every day to clear forests.

The risk is similar to the development of so-called "superbugs" that are resistant to antibiotics.

"If we do not put a stop to the drug-resistant malaria situation that has been documented in the Thai-Cambodia border, it could spread rapidly to neighbouring countries and threaten our efforts to control this deadly disease," Hiroki Nakatani, WHO assistant director-general, said in a statement.

The WHO plans to try and contain the spread of resistance with the help of a $22.5 million grant from the Bill & Melinda Gates Foundation. The prevalence of malaria has been reduced considerably over the past 50 years, but the disease still kills more than a million people every year.

Resistance along the Thai-Cambodia border started with chloroquine, followed by resistance to sulfadoxine-pyrimethamine and mefloquine, drugs used in malaria control several years ago.

The risk of resistance to any drug increases when it is used on its own, which is why the WHO recommends the routine use of artemisinin combination therapies, such as Novartis AG's Coartem.