"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, the WHO assistant director-general, said in a statement.
The prevalence of malaria has been reduced over the past 50 years, but the disease continues to kill 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 that have been used to treat malaria for several years.
The risk of resistance to any drug increases when it is used on its own. The WHO has therefore recommended the use of artemisinin combination therapies.