(CNN) -- A court in China's Yunnan Province on Wednesday rejected an appeal from six defendants convicted of killing 13 Chinese sailors, said the state-run Xinhua news agency.
Last month, the Intermediate People's Court of Kunming sentenced four of the men to death for the killings, which occurred in October 2011 on the Mekong River.
They were also ordered to pay compensation of 6 million yuan (about $960,000).
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One of the men is a drug lord identified as Naw Kham; the other three were members of his gang, Xinhua said.
Of the two other gang members, one received a death sentence with a reprieve and another was sentenced to eight years in prison.
Thai authorities discovered the murder scene after boarding two Chinese cargo ships that had come under gunfire.
They found nearly 1 million amphetamines and the 12 bodies, some with their hands bound. One sailor was missing.
The men had been charged with homicide, hijacking, kidnapping and drug trafficking.
The alleged gang was arrested in a joint operation by police from China, Laos, Myanmar and Thailand, according to the state-run China Daily.
Five of the six men are from Myanmar, Thailand and Laos; one is stateless, Xinhua said.
Naw Kham's gang colluded with Thai soldiers in the attack on the cargo ships, the Hua Ping and Yu Xing 8, Xinhua said.
Naw Kham had also directed several of his subordinates in the kidnapping of Chinese sailors and hijacking of cargo ships in exchange for ransom in early April 2011, the court said.
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CNN's Elizabeth Yuan contributed to this report.