Search crews flying over the Atlantic have reported finding debris spread over more than 90km of ocean, reinforcing the possibility that it broke up in the air.

A mini-submarine and a remote-controlled robot that explored the undersea wreckage of the Titanic are also being sent to help find the plane's flight recorders.

Ifremer, France's marine research institute, said on Thursday that a ship had left the Azores on Tuesday and could take up to 10 days to reach the search area.

'No explosion'

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But Brazil's government has discounted the idea of a mid-air explosion bringing down the plane, which was carrying 228 people from Rio de Janeiro to Paris when it dropped off radar screens on Monday.

Nelson Jobim, Brazil's defence minister, said the existence of large fuel slicks in the water probably meant there was no explosion.

"The existence of oil stains could exclude the possibility of a fire or explosion," he said at a news conference in Brasilia. "If we have oil stains, it means it wasn't burned."

But he said that was "just a hypothesis" and stressed that the reasons for the crash were still not known.

Investigators said a series of messages from the plane suggest massive failure before it plunged into the Atlantic.

Final messages

Final messages sent from the plane suggest the jet suffered a series of failures and could have broken apart as it passed through a thunderstorm before hitting the ocean off Brazil's northern coast on Monday, officials said.

The news came as the first Brazilian navy ship arrived in the area where the plane went down, in a bid to recover debris. Three more are due to arrive soon, the Brazilian navy said in a statement on Wednesday.

Brazil's air force has over the past two days spotted a plane seat, a life vest as well as a big chunk of what appeared to be aircraft fuselage and fuel slicks.

Brazilian navy divers will search a 5km strip of water strewn with parts from the plane about 1,200km northeast of the coastal city of Recife for bodies and also the plane's "black box" recorders in a bid to determine the plane's fate.

Three merchant vessels are already in the area after being diverted to help with the operation.

'Black boxes'

Flight AF 447 was heading from Rio de Janeiro to Paris when it went down early on Monday, in the worst loss of life in Air France's 75-year history.

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.

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.

The Airbus A330-200 was flying from Rio de Janeiro to Paris when radar contact was lost
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."

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.

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

Relatives' frustration

As investigation and recovery efforts continued distraught relatives who had prayed for a miracle gave up hope as experts were certain that all on board the flight had been killed.

"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 and on the plane, said.

However some relatives of those missing, presumed dead, expressed frustration that global media reported the latest developments in the search for the missing plane before family members had been informed by officials from the airline or the government.

"We don't want to hear from a reporter who is not there and who doesn't know the people who are there," Nelson Marinho, whose son was on the flight, told the Associated Press news agency on Wednesday.

So far no bodies have been sighted by the air force. Most of those feared dead are Brazilian or French but comprise a total of 32 nationalities.