A Lao Airlines plane carrying 44 people from the capital Vientiane to the southern town of Pakse crashed on Wednesday, killing all on board, a Thai foreign ministry spokesman said.
Laos officials informed Thailand that the plane carrying 39 passengers and five crew went down around 8km from the airport in Champasak province in southern Laos, said Thai foreign ministry spokesman Sek Wannamethee.
"I can now confirm, according to our reports, that all 44 people on board have died, including five Thai," he told the AFP news agency.
An official at the South Korean embassy in Bangkok told Yonhap news agency that three of their nationals were among the dead, and a minister told AFP that seven French citizens died as well.
Pakse is a hub for tourists travelling to more remote areas in southern Laos.
A Lao Airline official in Vientiane confirmed the crash, but was unable to provide further details of casualties. She told AFP that the plane had set off from Vientiene on time at 2:45pm [0745 GMT] and was supposed to arrive in Paske just over an hour later.
Thailand's English-language Bangkok Post reported that the plane had crashed into the Mekong River in bad weather at it tried to land at Pakse airport.
It said the crash had occurred at 4pm local time [0900 GMT].
Pictures on Thai television showed a small plane, half submerged in the river, with what appeared to be bodies lying on the banks.
Founded in 1976, the carrier operates a fleet of ATR-72 turboprop, Airbus A320 and Chinese-made MA60 planes, serving domestic airports and destinations in China, Thailand, Cambodia and Vietnam, according to its website.
It has a chequered safety record. Eight people died when a plane operated by the airline, then called Lao Aviation, crashed in remote mountains in the northeast of the country in October 2000.