Twelve Chinese crew members on two boats plying the Mekong River were killed by suspected drug traffickers who hijacked their ships to smuggle speed pills, news reports say.
The China Daily newspaper said that the boats were hijacked last Wednesday and that the bodies were found near Chiang Rai in northern Thailand on Friday and Saturday.
Most of the victims had been bound and blindfolded with adhesive tape and shot, the paper said. One crew member remains missing, it said.
The paper said the crew included two female cooks.
The Bangkok Post reported that local authorities had seized both boats after a gun battle with the hijackers and found cargo including speed pills worth 100 million baht (£2m), garlic, apples and fuel.
The Post cited Thai army officials as saying a gang run by suspected ethnic Shan drug trafficker Nor Kham was believed to be behind the attacks. It said the gang demands protection money from ships it hijacks on the Mekong and kills crew members who refuse to cooperate.
The boats are used to smuggle drugs from Burma to Thailand, it said.
China's Foreign Ministry said in an online statement that Chinese diplomats had asked for the Thai government's help in investigating last week's incident. The ministry said 11 crew were killed and two remained missing.
The Golden Triangle region, where the borders of Burma, Laos and Thailand meet, is notorious for the production and trafficking of heroin and other illicit drugs.
In April, three Chinese boats and 34 crew members were taken hostage by pirates along the Mekong in Burma but were safely rescued within days.