The rescue operation was also complicated by fast-moving currents in the muddy river and very poor visibility. Thai media reported that the river was about 8 meters (26 feet) deep in the area of the crash.
"If we could find (the plane) we would have found it already," the openly frustrated minister told reporters. "We're working very hard with our Thai friends."
Thailand, which lost five nationals in the crash, is deeply involved in the search, providing divers from the Thai navy and other skilled manpower that its poorer neighbor lacks.
France's accident investigation agency said in a statement that it was sending four investigators to help Laos with the probe into the cause of the crash. The statement said the team would work with technical advisers from ATR, the French-Italian manufacturer of the aircraft.
Lao Airlines has said the plane ran into extremely bad weather as it prepared to land at Pakse Airport. No further details on the investigation or circumstances of the crash have been released. The crash occurred about 7 kilometers (4 miles) from the airport.
Cambodian authorities said one of the plane's pilots was a 56-year-old Cambodian with more than 30 years' flying experience.
The passengers included foreign tourists and expatriates working in Laos.
Tourism has become a major source of income for Laos in the past decade. In 2012, the country received more than 3.3 million foreign tourists who generated total revenue of more than $513 million.
The area where the plane crashed is off the main tourist circuit in Laos but known for its remote Buddhist temples, nature treks and waterfalls.
According to the airline, 44 passengers and five crew were on the flight. The passengers included 16 Lao nationals, seven French, six Australians, five Thais, three Koreans, three Vietnamese and one person each from China, Malaysia, Taiwan and the United States. A person who had been listed as a Canadian was instead added to the list of Vietnamese.
The American and Malaysian were a married couple, Joel Babcock of Nebraska and his wife Angelin, the man's pastor Rev. Glen Wapelhorst said. Babcock attended the University of Nebraska-Lincoln from 2007 until 2010 before returning to Laos, where he'd lived as a child, the pastor said.
The Australians were a couple and their two young children and an aid worker based in Laos and his father.
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Associated Press writers Thanyarat Doksone and Jocelyn Gecker in Bangkok contributed to this report.