The bodies of 12 crew members were found near Chiang Rai in northern Thailand on Friday and Saturday after the ships were hijacked on Wednesday.
Another body was found in the same area early today, according to the Xinhua news agency.
Most of the victims had been bound and blindfolded with duct tape and executed.
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.
Chinese Foreign Ministry spokesman Liu Weimin told a news conference that China had suspended shipping from Yunnan down the Mekong and had sent a team to help investigate the killings.
He also said China had asked Thailand to increase security on the river.