France is sending its nuclear submarine SNA Emeraude to the area
A French nuclear submarine is being sent to help find an Air France plane which disappeared over the Atlantic.
French Defence Minister Herve Morin said the submarine had sonar equipment that could help locate the airliner's flight data recorders.
Meanwhile, it was revealed that debris salvaged from the sea was not from the Airbus jet that went missing on Monday.
Airbus has reissued guidelines to pilots after experts said the plane may have had false speed measurements.
A spokesman for the company said that a notice had been sent reminding Airbus crews worldwide what to do when speed indicators give conflicting read-outs.
Spokesman Justin Dubon said the readings meant that "the air speed of the aircraft was unclear".
He said that in such circumstances, flight crews should - if necessary - level off the plane and start troubleshooting procedures as detailed in operating manuals.
SEARCH FOR FLIGHT AF 447
1 June: Contact lost with plane over mid-Atlantic
2 June: First debris spotted from the air includes an airline seat. Brazilian defence minister says debris is from missing plane
3 June: More debris spotted, including a 7m-wide chunk of metal. Fuel slick seen on ocean surface
4 June: Buoys and pallet recovered from ocean said to be from plane. Officials later retract statement
The BBC's Tom Symonds says erratic speed readings could have been the result of heavy turbulence and might have caused the plane's automatic throttle to power up or down as it passed through heavy storms.
Meteorologists say the Air France Flight 447 had entered an unusual storm with 100mph (160km/h) updrafts that sucked water up from the ocean.
As the moisture reached the plane's high altitude it quickly froze in -40C temperatures. The updrafts would also have created dangerous turbulence, they say.
The Airbus A330 jet vanished over the Atlantic en-route from Rio de Janeiro to Paris on Monday with 228 people on board.
A small group of relatives of those on board the plane has gone to the north-eastern Brazilian city of Recife where the rescue operation is based. They are to be given a chance to tour the facility and to ask questions.
As the search continued on Friday, it was revealed that a wooden pallet and a fuel slick in the vicinity of the plane's last known position were not from the jet.
Brazilian air force official Brig Ramon Borges Cardoso contradicted earlier reports, saying "no material from the plane has been recovered".
The slick was most likely from a passing ship, he said.
Navy ships are reported to be scouring the ocean, about 1,100km (690 miles) north-east of Brazil's coast, in an effort to locate other debris spotted from the air on Tuesday and Wednesday.
In addition to the nuclear submarine, a French ship equipped with two deep-sea research mini-submarines is on its way to the area.
The mini-submarines will be looking for the plane's flight data recorders, which are believed to be sitting on the ocean floor up to 6,000m (19,685ft) underwater.
Three more Brazilian boats are expected to arrive in the area in the next few days.
French military spokesman Christophe Prazuck said the priority was looking for wreckage from the plane before it sinks or disappears.
French officials have said the flight data recorders, which could be deep under water, may never be found.
In another development on Friday, the Paris prosecutor's office opened a manslaughter investigation into the air crash.
It is a routine step taken by authorities in connection with the deaths of French citizens overseas.