"There is some early evidence that resistance to artemisinins may also be emerging on the Myanmar-Thailand border," said the WHO in a statement.
"There is also concern that resistance could spread from the Cambodia-Thailand border to Africa, as it did with anti-malaria drugs such as chloroquine and sulfadoxine-pyrimethamine in the 1960s and 1970s," it added.
Pascal Ringwald, who co-authored a WHO study into the issue, said the WHO is undertaking "complementary studies to confirm that it is indeed drug resistance. That should take a year."
In February 2009, anti-malarial drug resistance was confirmed by the WHO at the Cambodia-Thailand border.
But the latest WHO study found that some 10-20 percent of patients at the Myanmar-Thailand border continued to show signs of malarial parasites in the blood after a three day treatment with artemisinin combination therapy.
Likewise, at the China-Myanmar border, studies show that a quarter of patients who took oral artesunate monotherapy remained parasitaemic on the third day of treatment.
Similar signs were observed in a a province in Vietnam.
The WHO called for "careful monitoring" on the issue, and said that only 34 percent of malaria-endemic countries are complying with recommendations to monitor the efficacy anti-malarial drugs.
Half of the world's population is exposed to malaria which kills 860,000 people every year, according to the WHO.
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