They have confirmed that resistant strains of the malaria parasite on the border between Thailand and Burma, 500 miles (800km) away from previous sites.
Researchers say that the rise of resistance means the effort to eliminate malaria is "seriously compromised".
The details have been published in The Lancet medical journal.
For many years now the most effective drugs against malaria have been derived from the Chinese plant, Artemisia annua. It is also known as sweet wormwood.
In 2009 researchers found that the most deadly species of malaria parasites, spread by mosquitoes, were becoming more resistant to these drugs in parts of western Cambodia.
This new data confirms that these Plasmodium falciparum parasites that are infecting patients more than 500 miles away on the border between Thailand and Burma are growing steadily more resistant. Read More