Sao Paulo/Paris - Four days after an Air France jetliner plunged into the Atlantic Ocean, it remained unclear Friday where and why the aircraft went down.

The mystery over the location of the aircraft, which vanished early Monday with 228 people aboard, deepened after a Brazilian military spokesman said pieces of wreckage fished out of the sea did not come from the plane.

"We have so far recovered no piece of the crashed airplane," air force spokesman Ramon Cardoso said in the port city of Recife late Thursday local time.

A wooden palette plucked from the waters by a helicopter was "100 per cent" not from the Airbus A330-200 that disappeared while on flight from Rio de Janeiro to Paris.

In addition, French media reported that the kerosene slick spotted in the sea also did not stem from the aircraft, but came instead from a ship.

"The search goes on," Cardoso said.

French Junior Transport Minister Dominique Bussereau on Friday urged "extreme caution" in regard to the fragments fished out of the waters.

"I remind you that our airplanes and our ships have seen (no wreckage). It is our Brazilian friends who have seen things which they... said came from the plane," Bussereau told RTL radio.

Brazilian searcher planes had reported sighting pieces of wreckage in the sea, including what they said what a seat, which prompted Brazilian authorities to declare that there was "no doubt" the debris came from the missing aircraft.

The top priority, Bussereau said, "is to find the black boxes. But time is against us because their signal lasts only 30 days."

Each plane carries two black boxes, a flight data recorder and a cockpit voice recorder, each one equipped with an underwater locator beacon that sends out an ultrasonic signal equipment for 30 days.

Brazil has deployed 11 planes to the search zone, about 1,200 kilometres north-east of Recife, near the Saint Peter and Saint Paul Islets, a small, uninhabited archipelago.

France and the United States have also deployed airplanes, and a number of French and Brazilian ships are also involved in the search.

Bad weather is forecast for the area on Friday, which will hamper the search, Cardoso said.

France Inter radio reported Friday that relatives of some of the victims of the crash will be flown to Recife to observe the search operations.

The aircraft was carrying 216 passengers from 32 countries and a crew of 12 when it vanished mysteriously after apparently encountering bad weather.(dpa)