RECIFE, Brazil - Days after Air France Flight 447 vanished, an intensive international effort has failed to recover any confirmed wreckage and concern grew yesterday about whether searchers were even looking in the right place.

Air France, meanwhile, told its pilots in a memo obtained by the Associated Press that it was replacing instruments that affect flight speed in all its bigger jets. Investigators have focused on the equipment's possible role in the disaster.

Brazilian officials first reported Tuesday that military pilots had spotted wreckage from Flight 447 scattered across the ocean's surface, but pieces pulled out Thursday turned out to be unrelated to the plane.

Air Force Brig. Gen. Ramon Cardoso insisted yesterday that at least some of the debris spotted from the air - an airplane seat, a slick of kerosene, and other pieces - are from the plane that vanished Sunday with 228 people on board. The Brazilian air force also distributed images pinpointing where the material was found.

"This is the material that we've seen that really was part of the plane," Cardoso said.

But officials said extremely poor visibility had hampered efforts to guide ships to the spot where the debris was sighted, and France's Transportation Minister Dominique Bussereau said his country's searchers have found no signs of the Airbus A330.

"French authorities have been saying for several days that we have to be extremely prudent," Bussereau told France's RTL radio. "Our planes and naval ships have seen nothing."

A French Defense Ministry official, speaking on condition of anonymity because he was not authorized to speak on the matter, also questioned the Brazilian assertions, saying French teams "cannot precisely confirm the zone where the plane went down."

Aviation officials have said the crash investigation is increasingly focused on whether external instruments may have iced over, confusing speed sensors and leading computers to set the plane's speed too fast or slow - a potentially deadly mistake in severe turbulence.

Airbus has said the French agency investigating the crash found the doomed flight received inconsistent airspeed readings by different instruments as it struggled with severe turbulence in a massive thunderstorm.

The Air France memo says the company will finish replacing the instruments - known as Pitot tubes - in "coming weeks." It does not say when the replacement process started, and the company declined to comment on the advisory, saying it was meant for pilots only.

The L-shaped metal Pitot tubes jut from the wing or fuselage of a plane, and are heated to prevent icing. The pressure of air entering the tubes lets sensors measure the speed and angle of flight.

An iced-over, blocked, or malfunctioning Pitot tube could cause an airspeed sensor to fail, and lead the computer controlling the jet to accelerate or decelerate in a potentially dangerous fashion.

Questions about speed sensors are only one of many factors that investigators are considering. Automatic transmissions from the plane showed a chain of computer system failures that indicate the plane broke apart in midair.

The cause may be hidden on "black box" voice and data recorders that could lie miles deep on the ocean floor.