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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