The ATR twin-turbo flight QV301 with 44 people on board crashed into the Mekong River as it was coming in to land at the airport in Champassak province in southern Laos. Officials said the accident was caused by bad weather.
The plane left Wattay airport in Vientiane at 2.45pm and crashed at 4pm, the official said.
The crash occurred at Don Khong in Phonethong district near Pakse, reports said.
Laos officials informed Thailand that five Thai passengers were among 39 passengers and five crew on board the flight, said Thai Foreign Ministry spokesman Sek Wannamethee.
"We have been informed that all Thai passengers are dead," he said.
A foreign resident of Pakse said in an email message that the scene of the crash was horrendous.
"The Chinese Temple in front of my house has become an emergency centre," he said.
"I saw lifeless bodies laying about and other lifeless bodies being brought in, some connected to IV drips.
"It's complete chaos out front, as emergency vehicles grapple with usual traffic on this pot-holed, muddy stretch of road. Hundreds of people are loitering about, some curious, others presumably concerned for their loved ones.
"It's absolute horror."
Pakse, the capital of Champassak, is the largest city in southern Laos. It is opposite Ubon Ratchathani with the Chong Mek checkpoint in Sirindhorn district connecting the two countries.
The national airline of Laos operates domestic as well as international services to countries such as Cambodia, China, Thailand, Vietnam and Singapore. Its main base is the Wattay International Airport in Vientiane.
It operates 14 aircraft, including two ATR 73-600 of the type involved in the crash.
A list of passengers was released by the airline at about 9pm. It described the passengers as five from Thailand, five from Australia, three from Korea, seven from France, one from the US, one from China, one from Taiwan, one from Canada and two from Vietnam.
The five Thais are Yangyong Apaanan, Nipol Mengsee, Kamueng Chartkasamchai, Phakkhawat Atiratanachai and Veekij Busarawutilanu.
The Australians are Gordon Bruce Creighton, Michael Creighton, Phoumalaysy Rhodes, Manfred Gavin and Geoffre Gavin Rhodes.
The Koreans are Lee Hongjig, Lee Jaesang and Lee Gangpil.
The French citizens are Helene Andrew Detable, Guy Henri Vimont, Jean Luc Salelles, Francoise Detable, Dominique Vimont, Patricia Debord and Bruno Debord.
The American is Joel Babcock.
The Chinese is Rui Gan.
The Taiwanese is Chang Yu Liu.
The Canadian is Thilieu Dao.
The Vietnamese are Le Hue and Thigan Voung.
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