France held religious ceremonies to remember the missing on Wednesday, including one in Paris' Notre-Dame cathedral attended by Nicolas Sarkozy, the country's
president.

Three days of national mourning began in Brazil on Tuesday.

'Black boxes'

In depth


 Article: Tearful relatives wait for news of missing jet
 Timeline: Air France accidents 
 
Profile: Airbus A330-200 

Videos:
 Route map of Air France 447
 
Brazil passengers await fate of Air France plane
 Air France jet goes missing over Atlantic

Admiral Domingos Nogueira of the Brazilian navy said the hardest task would be finding the flight data and voice recorders that hold clues to why the plane disappeared from radar screens.

The "black boxes", which are mounted in the tail of an aircraft, record the performance and the condition of the plane in flight and are crucial for air crash investigations.

"The ships are equipped to arrive and pick up pieces of the Airbus," Nogueira said.

"Each ship has two divers on board and smaller ships to throw into the ocean to try and get pieces."

Helicopters would then be used to take wreckage of the Airbus A330 from the ships to a base on the Brazilian archipelago of Fernando de Noronha, 700km from the crash site.

Significant depth

Officials said the boxes needed to identify the causes of the crash could be on the ocean floor at a depth of between 2,000 and 3,000 metres.

Black box: key to air crash investigations 

Modern black boxes, which are actually orange in colour, record up to 300 factors of flight including:

 Airspeed and altitude
 Heading and vertical acceleration
 Aircraft pitch
Cockpit conversations
Radio communications

The recorders are designed to send signals for up to 30 days, but many do not float well.

One expert said it could be among the hardest recoveries since the decades-long search to find the Titanic.

The US has dispatched specialist radar equipment to the area to search for the recorder, and France is also sending a research ship equipped with two mini-submarines to the disaster area.

Luiz Inacio Lula da Silva, Brazil's president, said he was confident that the boxes would be located.

"I think a country that can find oil 6,000 metres (19,685 feet) under the ocean can find a plane 2,000 metres (6,500 feet) down," he said on Tuesday.

Speculation

There is speculation that the plane ran into stormy weather before crashing.

Authorities are at a loss to explain how a storm could have caused the plane, being flown by three experienced pilots, to crash without sending a distress call.

Brazil's air force last had contact with Flight AF 447 at 01:33 GMT on Monday when it was 565km from its coast. The last automated signals, which reported an electrical failure, were received about 40 minutes later.

Flight AF 447 was en route to Paris from
Rio De Janeiro when it went down
One theory is that a lightning strike or severe weather set off a series of failures. But lightning routinely hits planes and could not alone explain the downing, aviation specialists have said.

Distraught relatives who had prayed for a miracle gave up hope as experts were certain that all aboard died on the flight.

"I just want to find my son's body so that he can have a dignified burial," Aldair Gomes, the father of Marcelo Parente, who was the head of the Rio mayor's cabinet, said.

So far no bodies have been sighted by the air force. Most of the missing people are Brazilian or French but they include a total of 32 nationalities.