An aviation official in Paris said they were "very worried".
"The plane disappeared from the screens several hours ago. It could be a transponder problem, but this kind of fault is very rare and the plane did not land when expected."
Air France said it had no further information on the whereabouts of Air France flight AF 447, which it said had 216 passengers and 12 crew on board.
Gabriel Elizondo, Al Jazeera's correspondent in Sao Paulo, said the Brazilian air force had mounted a search.
"The Brazilian air force ... has apparently said they are searching near an island called Fernando de Noronha, which is a Brazilian island about 340km off the northeast of the Brazilian coast," he said.
"But it's important to note that this plane has only gone missing at this point."
Authorities have set up a crisis centre at Charles de Gaulle airport, with Dominique de Bussereau, the French junior transport minister, due to arrive.