Laos on Friday said rescuers had recovered at least 17 bodies after a plane plunged into the Mekong River in the country's worst air disaster, with one corpse found miles downstream from the crash site.

All those on board the plane died when the Lao Airlines turboprop ATR-72 went down in stormy weather on Wednesday.

Of the forty-nine passengers and crew, more than half of them were foreigners from nearly a dozen countries.

Volunteers searched the river on boats of all sizes, mustered for the grim task of plucking the dead from the turbulent waters and its muddy banks.

The aircraft sank to the bottom of the river. Rescuers said that recovering the wreckage would be an extremely difficult task, complicated by raging currents.

Soubinh Keophet, a former national footballer and volunteer with a Laos rescue foundation, said one body was discovered as far as 30 kilometres (19 miles) from the crash site.

"We travelled 50 kilometres (31 miles) along the river and found four bodies," he said, after he pulled a recently discovered limb from the water.

"Although they know the location of the crash, they still can't find the main body of the plane because it has broken up into small parts and spread everywhere and the current is very strong," he added.

A witness recounted a desperate attempt to rescue passengers after the plane dived nose first into the river.

"The front of the plane went under but the tail was still floating. There was smoke coming out," Pham Quang Nhat told Vietnam's Tuoi Tre newspaper.

"We waited to be sure the plane would not explode then some people used boats to approach the plane, climbing on the wings hoping to find survivors. But we couldn't get inside," the Vietnamese worker said.

Sommad Pholsena, Laos minister of public works and transport, told reporters that 17 bodies had been found so far.

"We have to investigate about the cause of the accident but initially I think it was caused by bad weather," he said.

A large Laos naval vessel, several smaller Thai and Laos rescue boats, dinghies and a jet ski were seen on the waters on Friday.

Thai Transport Minister Chatchat Sitthipan said near the scene that the rescue operation was being led by local authorities with the support of the Thai navy, airforce and volunteer rescue teams.

"We stand ready to support Laos with whatever it needs," he said.

The flight from the capital Vientiane was carrying 44 passengers and five crew, including 28 foreigners, when it crashed near Pakse airport in Champasak province, according to officials.

Rows of wooden coffins were seen at a mortuary in Pakse, which is a hub for tourists travelling to more remote areas in southern Laos.

Lao Airlines said the aircraft hit "extreme" bad weather while witnesses described seeing the plane buffeted by strong winds caused by tropical storm Nari.

According to an updated passenger list from the airline, there were 16 Laotians, seven French travellers, six Australians, five Thais, three South Koreans, three Vietnamese, and one national each from the US, Malaysia, China and Taiwan.

Australia said a family of four was among its nationals feared dead.

The pilot was a Cambodian national said to have "many years" of flying experience.

French-Italian aircraft manufacturer ATR said the twin-engine turboprop aircraft was new and had been delivered in March.

Witnesses recounted seeing the plane in trouble before it came down.

"I heard a boom! A sound like a bomb going off. There was smoke and flames before it crashed," local village chief Buasorn Kornthong, 37, told AFP.

Founded in 1976, Lao Airlines serves domestic airports and destinations in China, Thailand, Cambodia and Vietnam.

Impoverished Laos, a one-party communist state, has seen 29 fatal air accidents since the 1950s, according to the Aviation Safety Network.

Previously its worst air disaster was in 1954 when 47 people died in an Air Vietnam crash near Pakse, the organisation said.