Malaria parasites resistant to key drugs have spread rapidly in South East Asia, researchers from the UK and Thailand say.
The parasites have moved from Cambodia to Laos, Thailand and Vietnam, where half of patients are not being cured by first-choice drugs.
Researchers say the findings raise the “terrifying prospect” drug-resistance could spread to Africa.
However, experts said the implications may not be as severe as first thought.
In some regions, 80% of malaria parasites were drug resistant.
“This strain has spread and has become worse,” Dr Roberto Amato, from the Wellcome Sanger Institute, told BBC News.
Huge progress has been made towards eliminating malaria. However, the development of drug resistance threatens that progress.
“This highly successful resistant parasite strain is capable of invading new territories and acquiring new genetic properties, raising the terrifying prospect that it could spread to Africa, where most malaria cases occur, as resistance to chloroquine did in the 1980s, contributing to millions of deaths,” Prof Olivo Miotto, from the Wellcome Sanger Institute and University of Oxford, said.
But there are alternative drugs that can be used instead.
“With the spread and intensification of resistance, our findings highlight the urgent need to adopt alternative first-line treatments”, Prof Tran Tinh Hien, from the Oxford University Clinical Research Unit, in Vietnam, said.