International scientists say they have found the first evidence of resistance to the world's most effective drug for treating malaria.They say the trend in western Cambodia has to be urgently contained because full-blown resistance would be a global health catastrophe.Drugs are taking longer to clear blood of malaria parasites than before.This is an early warning sign of emerging resistance to a disease which kills a million people every year.Until now the most effective drug cleared all malaria parasites from the blood within two or three days but in recent trials this took up to four or five days.The BBC's Jill McGivering, reporting from Cambodia, says it is unclear why the region has become a nursery for the resistance - but the local public health system is weak, and the use of anti-malaria drugs is not properly controlled.Drug defenceThe 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, have reported seeing the disturbing evidence that the drugs are becoming much less effective.There is particular concern because previous generations of malaria drugs have been undermined by resistance which started in this way, in this part of the world, our correspondent reports.Professor Nick Day is the director of the Mahidol-Oxford Tropical Medicine Research Unit, one of the teams involved in the research."Twice in the past, South East Asia has made a gift, unwittingly, of drug resistant parasites to the rest of the world, in particular to Africa," he said."That's the problem. We've had chloroquine and SP resistance, both of which have caused major loss of life in Africa," in said in reference to earlier generation anti-malarial drugs."If the same thing happens again, the spread of a resistant parasite from Asia to Africa, that will have devastating consequences for malaria control," he said.Health systemsCambodia has long been a laboratory for malaria investigators and a nursery of anti-malaria drug resistance.Alongside a weak public health system and poorly-controlled drug use, there are many fake drugs, produced by international criminals.These fakes often contain a small amount of the real drug to fool tests, which can also help to fuel resistance.Those working to control malaria are calling for urgent action to contain this emerging resistance.If it strengthens and spreads, they warn, many millions of lives will be at risk. About half the world's population faces exposure to the disease.Credit: BBC