The Ministry of Public Works and Transport, which operates the airline as a state enterprise, said 44 passengers and five crew members were aboard flight QV301 from the Lao capital, Vientiane, to Pakse in the country's south. Earlier reports had said there were 39 passengers on board.
"Upon preparing to land at Pakse Airport the aircraft ran into extreme bad weather conditions and was reportedly crashed into the Mekong River," the ministry said in a statement.
It said there was no word of survivors. The airline flies an ATR 72-600 twin-engine turboprop plane on the 467-kilometer (290-mile) route. The aircraft is configured with 68-74 seats, said maker ATR.
Thai Foreign Ministry spokesman Sek Wannamethee said his country's embassy in Vientiane was informed that the plane crashed 7-8 kilometers (4-5 miles) from the airport at Pakse.
A passenger manifest faxed by the airline listed 44 people: 17 Lao, seven French, five Australians, five Thais, three Koreans, two Vietnamese and one person each from Canada, China, Malaysia, Taiwan and the United States. Korean, French and Thai officials confirmed the totals for their nationalities.
The Lao government said the airline "is taking all necessary steps to coordinate and dispatch all rescue units to the accident site in the hope of finding survivors."
However, a statement from Australia's Department of Foreign Affairs and Trade said "Lao authorities have told our embassy in Vientiane they do not expect any survivors."
The Lao transport ministry statement said the crash is being investigated and the airline hoped to announce its findings on Thursday. A Lao Airlines employee contacted by phone at Vientiane's Wattay airport said a news conference would be held Thursday.
The aircraft's maker, ATR, issued a statement from its headquarters in Toulouse, France, declaring that it will provide full assistance under international aviation rules to the investigation of the crash, working with the French safety investigation body. It said the Lao Airlines plane had been delivered from the production line in March this year.
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