HANOI, Vietnam (AP) — Scientists say Vietnam's main malaria treatment is failing at an alarming rate because of a highly drug-resistant superbug that has spread into the southern part of the country from western Cambodia.

The scientists said in a letter published Thursday in the Lancet Infectious Diseases journal that the spread of the superbug across the entire Mekong subregion is a serious threat to malaria control and eradication efforts.

The two scientists from Mahidol Oxford Tropical Medicine Research Unit in Thailand urged an urgent response to the public health emergency.

Vietnam's health ministry had said in April that malaria resistant to artemisinin has been reported in five provinces and was threatening to spread nationwide.

Resistance to the treatment has previously been detected in parts of Thailand, Myanmar and Laos.