At least seven French citizens were among those killed when a Lao Airlines plane carrying 44 people crashed in southern Laos on Wednesday, Foreign Minister Laurent Fabius said.
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.
"I have just learned with deep shock and great sadness of the air disaster that took place in southern Laos in which at least seven of our compatriots were killed," Fabius said in a statement.
"My first thoughts are with the families and friends of the victims," said Fabius, who said France was rushing embassy officials to the site of the crash.
He said a crisis cell had been set up to help the families of the victims.
"Every effort will be made to support and assist the families of our compatriots in dealing with this terrible tragedy," Fabius said.
Officials in Thailand said all 44 people aboard the Lao Airlines plane died in the crash. According to reports the aircraft came down in the Mekong River in bad weather shortly before it was due to land at Pakse airport.
The plane was its way from the capital Vientiane when it crashed at about 16:00 local time (09:00 GMT).
Thai Foreign Ministry spokesman Sek Wannamethee said the flight crashed seven to eight kilometres short of the international airport of Pakse.
Images broadcast on Thai TV show a plane half-submerged in the river, AFP news agency reports.
It is believed there were 39 passengers on board and five crew members.
The crashed plane was an ATR-72 belonging to the state-owned airline, the agency said.
An official at the South Korean embassy in Bangkok told Yonhap news agency that three South Koreans were also among the dead.
State-owned Lao Airlines confirmed the crash in a statement on its official Facebook page, in which it expressed "our 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.
Laos has proved a popular destination for French tourists over the years.
The country has had 29 fatal air accidents since the 1950s, according to the Aviation Safety Network, whose data showed that the country's safety record has improved dramatically in the last decade.
(Pakse, where the Lao Airlines plane came down on Wednesday. Image Google Maps)