Authorities were trying to ascertain whether all 13, who had each suffered gunshot wounds, were members of the boats' crews, he said. One body was found on a ship while the rest were retrieved from the river.
"We believe these two cargo boats were attacked by a drug group who were active in the Golden Triangle," Sermsak told AFP, adding that 920,000 amphetamine tablets were discovered on the boats.
Sermsak said the gang could have hijacked the boats further north on the river, between Myanmar and Laos, and attempted to force the crew to carry illicit cargo, but that a shootout appeared to have broken out.
China's ministry of foreign affairs said it was suspending shipping on the Mekong on Monday in response to the attack on the cargo ships "Hua Ping" and "Yu Xing 8" on October 5.
Some Chinese ship operators have asked their Chinese crew to return to China overland from Thailand despite pending deliveries.
Olivier Evrard, anthropologist at the French Institute of Research for Development (IRD), said the Mekong in the Golden Triangle has long been "an area of fluvial piracy".
He said continuing piracy in the area was explained by a "low degree of political control" and a "high volume of commercial flux".
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