Prazuck said that the search of the rugged seabed at a depth of thousands of metres in strong ocean currents would be complicated and could take weeks.
He said: "Up to now, the time frame for the search for victims and debris has been of the order of days or a week.
"Here, at the very least, it's going to be of the order of weeks or months."
The spokesman pointed out that searchers had taken two weeks to locate the "black box" recorders after the crash of a Boeing 737 at Sharm-el-Sheikh in Egypt in 2004, despite much easier conditions.
"That aircraft crashed very close to the coast, there was no doubt about where the accident happened and it took 15 days to recover the black box," Prazuck said.
"Here the accident happened 1,000 km from the coast. The situation is very complex."
All 228 people aboard Air France flight AF 447 from Rio de Janeiro to Paris were believed to have died when the Airbus A330 crashed into the sea after flying into stormy weather more than a week ago.
Brazilian military search teams have recovered 41 bodies and moved some of them to the archipelago of Fernando de Noronha off Brazil's northeastern coast, which is being used as a base for the search operations.
Several pieces of wreckage have also been found but a full understanding of the accident will depend on the recovery of the flight recorders.
The plane sent 24 automated messages in the final minutes before it disappeared on June 1, detailing a rapid series of systems failures.
Speed sensors that gauge how fast an aircraft is flying have become the focus of the investigation after some of the messages showed they provided inconsistent information to the pilots.
Alter, a French pilots union, on Tuesday advised its members to stop flying certain types of Airbus until Air France replaced the sensors.