"While formal confirmation must still be obtained by recovering debris and carrying out technical analysis, there is no longer any room for doubt," Capt Prazuck told Agence France-Presse news agency.
An aeroplane seat, an orange buoy and signs of fuel have been spotted in the ocean.
Recording devices
If it is confirmed that all 228 people on Flight AF 447 are dead, it will be the worst loss of life in Air France's history.
"The last bit of hope that we had no longer exists," said Aldair Gomes, whose son was a passenger on the plane.
"Before, a lot of us were hoping that the plane could have landed on an island or something like that, but no more."
The first Brazilian naval vessel is due to arrive in the zone where the debris was found on Wednesday morning. The navy says the weather in the area is poor.
It is hoped that the ship will be able to recover wreckage that will give some clue as to why the plane fell from the sky.
Three merchant vessels are already in the area after being diverted to help with the operation.
Search teams are hoping to locate the plane's cockpit voice and data recorders, which will give the clearest information about what happened.
But Mr Jobim warned that recovering the devices could be difficult.
"It could be at a depth of 2,000m or 3,000m [6,500ft-9,800ft] in that area of the ocean," he said.
Our only certainty is that the plane did not send out any distress call
The US has despatched specialist radar equipment to the area to hunt for the recorder, and France is also sending a research ship equipped with two mini-submarines.
French Prime Minister Francois Fillon said the teams were "in a race against the clock".
He told parliament that the cause of the plane's loss had still to be established.
"Our only certainty is that the plane did not send out any distress call but regular automatic alerts for three minutes indicating the failure of all systems," he said.
Experts remain puzzled that there were no radio reports from the Airbus and they say that such a modern aircraft would have had to suffer multiple traumas to plunge into the sea, the BBC's Adam Mynott reports from Paris.
Most of the missing people are Brazilian or French but they include a total of 32 nationalities. Five Britons and three Irish citizens are among them.
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