Relatives of the murdered Chinese sailors are overwhelmed with grief during a memorial service in Chiang Saen in northern Thailand on Thursday night. [China Daily]

BEIJING - China on Thursday demanded swift action from three Southeast Asian countries as they investigate an attack on the Mekong River that claimed the lives of 12 Chinese sailors near the Thai-Myanmar border. One sailor remains missing.

In a related development, a Chinese patrol ship will escort 164 sailors and family members, who have been stranded on the Mekong since the attack, home on Friday.

The 12 victims were crew members of two cargo ships attacked earlier this month where the borders of Thailand, Myanmar and Laos meet.

A probe into the attack has reportedly stalled due to chaotic border management and lax security measures in the region.

Vice-Foreign Minister Song Tao summoned envoys from the three countries and urged their authorities to "step up their investigations, get to the bottom of the matter as soon as possible, report their findings to China in a timely manner and ... severely punish the culprits.

"The Chinese government values the life and safety of every Chinese citizen and demands a thorough probe of what happened," Song said.

The government is shocked and deeply saddened by the incident and strongly condemns the atrocity, Song said.

"The murderers must be brought to justice."

On Oct 5, 12 Chinese sailors were murdered after two cargo ships, Hua Ping and Yu Xing 8, were hijacked on the Mekong River in Thailand.

The site of the attack is in the Golden Triangle, a region notorious for drug smuggling.

Song urged the countries "to take effective measures ... to avoid similar incidents from happening again".

He asked the three nations to protect Chinese ships and citizens currently stranded in Thailand's Chiang Saen port. China will work with all sides to enforce security on the Mekong, he said.

China will send a patrol ship on Friday to pick up the stranded sailors and he hopes that the three countries will provide every assistance, Song said.

The envoys from the three nations offered their condolences. All three countries will coordinate with China to carry out a thorough investigation, the envoys said.

Relatives of the deceased sailors and Yunnan provincial officials arrived on Thursday at a hospital in Chiang Saen where the bodies were kept, Li Hui, a senior publicity official of Yunnan province, told China Daily.

DNA tests will be conducted to confirm the identities of the bodies.

Mourning ceremonies were planned at a temple there for Friday morning before the relatives return to Xishuangbanna, in Yunnan, on Friday afternoon.

The families of the deceased sailors have each received 100,000 yuan ($16,000) in compensation from insurance companies.

Wang Lina, 24, a daughter of one of the sailors, said her father, Wang Guichao, had talked to her about the dangers of working on the Mekong River.

"My father worked all year round on the boat, except for the Spring Festival," she said.

She said her father earned a meager 2,000 yuan a month, and was the only bread winner for a family of six, including a 23-year-old son in college and an 80-year-old mother, who has still not been informed of his death.

The stranded Chinese sailors and their family members could return on Friday, officials said on Thursday.

A plan for their return is in place, said Shi Minghui, head of a team of officials dispatched by the government of Yunnan province to Chiang Saen.

The sailors and their families, on 26 boats, have stayed on board since the attack.

Thai authorities said earlier this week that they believed the two ships were attacked by a drug group active in the Golden Triangle, adding that 920,000 amphetamine tablets were discovered on board.

All the bodies discovered had gunshot wounds with their hands tied behind their back.

Yang Deyi, the captain of Yu Xing 8, remains missing.

About 100 Chinese cargo ships sail annually on the Mekong, carrying about 400,000 tons of goods.

A number of non-fatal attacks on ships were reported in recent years.

An attack on 18 Chinese tourists on the Mekong River in August forced Chinese tourism firms to suspend operations in the area.

Experts ruled out any diplomatic fallout between China and the three countries.

Song Qingrun, a researcher at the China Institute of Contemporary International Relations, said the murders were a criminal case with no bearing on diplomatic relations.

Song added that the incident highlighted that the security situation in the Golden Triangle area needed to be reviewed.

China might take a more active role in promoting cross-border security mechanisms, Song Qingrun said.

China has announced it is suspending shipping on the Mekong in response to the attack.

Xinhua, AFP and Reuters contributed to this story.