All 44 passengers and five crew members were presumed dead although a search was still under way for survivors in the river, the state-owned airline reported.
The ART twin-turbo plane flight QV301 crashed into the Mekong River as it was coming in to land at the airport in Champassak province in southern Laos after flying into a rainstorm, the airline said.
Pakse is a hub for tourists travelling to more remote areas in southern Laos.
The passenger list consisted of 17 Laotians, seven French nationals, five Australians, five Thais, three South Koreans, two Vietnamese, one Chinese, one Canadian, one Myanmar national, one person from Taiwan and one American.
Relatives of the passengers had been informed of the accident, Lao Airlines said.
The plane left Wattay airport in Vientiane at 2.45 pm (7.45 am GMT) and crashed at 4 pm local time (9 am GMT).
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Laos officials earlier said that the plane carrying 39 passengers and five crew members went down around eight kilometres (five miles) from the airport in Champasak province in southern Laos, according to 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 said.
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A television station in Thailand also said 39 people had been killed. Pictures on Thai television showed a plane, half submerged in the river, with what appeared to be bodies lying on the banks.
Meanwhile French Foreign Minister Laurent Fabius said that at least seven French citizens were among those killed in the Lao Airlines plane crash.
In a statement, Fabius said he had learned of the deaths with "deep shock and great sadness" and that France was rushing embassy officials to the site of the crash.
An official at the South Korean embassy in Bangkok told Yonhap news agency that three South Koreans were also among the dead.
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State-owned Lao Airlines confirmed the crash in a statement on its official Facebook page, in which it expressed "condolences to family, friends, colleagues and relatives" of the passengers.
It said the aircraft hit "extreme" bad weather and had crashed into the Mekong River.
"There were no news of survivors at this time," it said, but did not confirm the number of deaths.
"Lao Airlines is taking all necessary steps to coordinate and dispatch all rescue units to the accident site in the hope of finding survivors and at the same time informing relative of the passengers," the English language statement said.
Thai newspaper Thairath News earlier reported that the plane could have exploded in the air.
A spokesman from aircraft manufacturer ATR in France confirmed the crash and said that the state-owned Lao Airlines flight was one of its twin-engine turboprop ATR-72 planes. He said Lao Airlines has a fleet of six ATR-72 planes.
An official at the Vietnamese Embassy in Laos told AFP on condition of anonymity that all on board the plane had been killed.
Voice of Russia, Bangkok Post, Reuters, AFP