At least 11 Chinese sailors were killed when their ships were attacked on the Mekong River between Thailand and Myanmar, the Chinese Foreign Ministry said, voicing concern about what media reports said appeared to be an assault by drug smugglers.

The sailors were on two cargo ships attacked on Wednesday in the "Golden Triangle" of the Mekong, a region of Southeast Asia notorious for narcotic production, the Foreign Ministry said on its website late on Sunday.

As well as the 11 sailors killed, two were missing, said the ministry.

But officials in Yunnan, the Chinese province next to Thailand and Myanmar, later said 12 were killed and one was missing, the China News Service reported.

Chinese sailors had been attacked and killed on the river before, it added, without giving details.

China's growing presence in Asia, Africa and other parts of the world has prompted attacks, kidnappings and hijackings, and the issue has become a sensitive one for Chinese officials, who do not want to appear weak in protecting nationals.

Beijing has "asked the countries concerned to take effective measures to strengthen protection of Chinese vessels and crew on the Mekong River", the Foreign Ministry said.

Crew on another boat that witnessed the attack said eight or so armed men stormed the two ships, and in the days since, Thai police have pulled bodies of the sailors from the river, said the China News Service.

The attackers appeared to be drug smugglers who sought to use the seized ships to traffic narcotics, said the report, citing Thai media accounts.

The Golden Triangle region - where the borders of Myanmar, 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 Myanmar but were safely rescued within days.

The Mekong snakes from China into Southeast Asia, where it forms part of Thailand's border with Myanmar and Laos, and in 2001 the four countries signed an agreement to regularize shipping on the river. The 4,900km river also flows through Cambodia and Vietnam

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The Mekong snakes from China into Southeast Asia, where it forms part of Thailand's border with Myanmar and Laos

Al Jazeera