A passenger plane carrying 116 people is feared to have crashed on a flight from Burkina Faso to the Algerian capital Algiers.
Contact with the Air Algerie flight was lost over the Sahara as it crossed Mali in bad weather, officials said.
French Foreign Minister Laurent Fabius said the plane, which has 51 French citizens aboard, "probably crashed".
There have been reports of wreckage being found but they are in locations up to 400km (250 miles) apart.
However, the French military, which is leading the search operation in the desert, has not confirmed finding any remains of the plane.
Two UN helicopter crews who took part in the search also failed to locate wreckage before nightfall.
Contact with Flight AH 5017, chartered from Spanish airline Swiftair, was lost about 50 minutes after take-off from Ouagadougou, Air Algerie said.
The pilot had contacted Niger's control tower in Niamey to change course because of a sandstorm, officials say.
BBC West Africa correspondent Thomas Fessy says the route is well used by French travellers.
Speaking in Paris, Mr Fabius said: "Despite intensive search efforts no trace of the aircraft has yet been found. The plane probably crashed."
He said two French Mirage fighter planes were scouring the area.
French President Francois Hollande cancelled a planned visit overseas and said every effort was being made to find the plane.
"The search will take as long as needed," he told reporters.
Earlier, an Algerian official told Reuters that the plane had crashed, but gave no further details.
France's civil aviation body said crisis centres had been set up at airports in Paris and Marseille.
Burkina Faso authorities said the passenger list comprised 27 people from Burkina Faso, 51 French, eight Lebanese, six Algerians, two from Luxembourg, five Canadians, four Germans, one Cameroonian, one Belgian, one Egyptian, one Ukrainian, one Swiss, one Nigerian and one Malian.
The six crew members are Spanish, according to the Spanish pilots' union.UN troops in Mali say they understand the plane came down between Gao and Tessalit, the BBC's Alex Duval Smith in the Malian capital Bamako reports.
She says the search area is vast, with few roads, and there is rebel activity. Added to that, sandstorms make visibility in the Sahara poor for at least a day, she adds.
"In keeping with procedures, Air Algerie has launched its emergency plan," Air Algerie officials, quoted by APS news agency(in French), said.
Algerian Prime Minister Abdelmalek Sellal reportedly told Algerian radio: "The plane disappeared at Gao (in Mali), 500km (300 miles) from the Algerian border."
Burkina Faso Transport Minister Jean Bertin Ouedraogo said the plane sent its last message at around 01:30 GMT, asking air traffic controllers in Niger to change its route because of bad weather.
In a statement (in Spanish), Swiftair said that the aircraft was a McDonnell Douglas MD-83 and that they were unable to establish contact with it.
An Algerian official had previously told Reuters that the plane was an Airbus A320.
An unnamed Air Algerie company source, speaking to AFP news agency, said: "The plane was not far from the Algerian frontier when the crew was asked to make a detour because of poor visibility and to prevent the risk of collision with another aircraft on the Algiers-Bamako route."
"Contact was lost after the change of course."
Flight AH 5017 flies the Ouagadougou-Algiers route four times a week, AFP reported.
In February a military plane in Algeria crashed, killing 77 people on board.
The Hercules C-130 crashed into a mountain in Oum al-Bouaghi province, en route to Constantine, in bad weather conditions. Only one person on board survived.
Source;@bbc.co.uk.news

RIP to the deceased