PARIS- A French submarine with advanced sonar equipment was due to begin searching on Wednesday for the flight recorders of an Air France airliner that crashed into the Atlantic last week, the French military said.

The nuclear-powered submarine Emeraude was sent to the area to help recover the black box flight recorders, which may contain clues to explain the disaster and which are believed to lie deep on the ocean floor.

The Emeraude will begin its patrol during the morning in an initial search zone of 36 kilometers by 36, Christophe Prazuck, a spokesman for the French military said. He said the search zone would be changed daily.

If the recorders are found, unmanned submarines from the Pourquoi Pas, a French exploration and survey ship also deployed to the area, could be used to bring them in.

Prazuck cautioned that the search of the rugged seabed at a depth of thousands of meters in strong ocean currents would be complicated and could take weeks.

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, he told LCI television.

He noted 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, he said. Here the accident happened 1,000 kilometers 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 doomed plane sent 24 automated messages in its final minutes 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 data to the pilots.

(Writing by James Mackenzie; editing by Andrew Dobbie)