A French submarine with sonar equipment has begun aiding the search for "black box" flight recorders from the Air France plane that crashed into the Atlantic last week, killing 228 people.
The nuclear-powered Emeraude was sent to the area to help recover the recorders, which may contain clues to explain the crash and which are believed to lie deep on the ocean floor.
Christophe Prazuck, a spokesman for the French military, said on Wednesday: "The Emeraude will begin its patrol ... in an initial search zone of 36km by 36km."
The search area will be changed daily and if the recorders are found, unmanned submarines from the Pourquoi Pas, a French exploration and survey ship deployed in the area, could be used to bring them up.
'Complex' search
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."
Stormy weather
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.