International scientists say they have found the first evidence of resistance to the world's most effective drug for treating malaria.
They say the emergence of the trend in western Cambodia has to be urgently contained, because full blown resistance to malaria would be a global health catastrophe.
The artemesinin family of drugs is the world's front-line defence against the most prevalent and deadly form of malaria.
Two teams of scientists working on separate clinical trials say they are seeing disturbing evidence that the drug is becoming much less effective.
Until now it cleared the blood of malaria parasites within two or three days.
But in both these trials clearance took much longer, sometimes four or five days.
This is an early warning sign of emerging resistance to a disease which kills a million people every year.