"All possibilities must be examined. We cannot, by definition, exclude a terrorist attack, because terrorism is the main threat for all Western democracies," he said, quoted by AFP news agency.
Clashing fronts
France believes there is little or no hope of finding survivors from among the 228 passengers and crew aboard the flight.
Vessels from France, Spain, Senegal and Brazil are involved in the search, while the United States is said to be offering help with satellite reconnaissance.
But French military spokesman Christophe Prazuck said storms and heavy clouds were severely limiting visibility.
"We didn't find anything, but the weather was terrible, with what we call a tropical convergence front," he said, quoted by AFP news agency.
He was describing a situation in which weather fronts from the northern and southern hemispheres clash violently.
Plane crews have narrowed their search to a zone half-way between Brazil and west Africa, said Pierre-Henry Gourgeon, chief executive of Air France, late on Monday.
Their work may be aided by the Airbus's Argos beacons, which will emit signals for several days, he added.
Up to a dozen reports of electrical failures were sent from the plane before it vanished over the ocean.
French officials believe it may have been disabled by a storm.
Expert explains how the search is conducted
Most of the missing people are Brazilian or French but they include a total of 32 nationalities. Five Britons and three Irish citizens are among them.
One of the Brazilians on board was Pedro Luis de Orleans e Braganca, a direct descendent of the last Brazilian emperior, Dom Pedro II, a spokesman for the family said.
French President Nicholas Sarkozy said he had told friends and relatives waiting at the Charles de Gaulle "the truth" - that the chances of finding anyone alive were "very small".
If no survivors are found, it will be the worst loss of life involving an Air France plane in the firm's 75-year history.
'Bright spots'
Cap Prazuck said two French military aircraft were searching the suspected area where the plane was lost.
TIMELINE Flight AF 447 left Rio at 1900 local time (2200 GMT) on SundayAirbus A330-200 carrying 216 passengers and 12 crewContact lost 0130 GMTMissed scheduled landing at 1110 local time (0910 GMT) in Paris Timeline of Flight AF 447Air disasters timeline
One of them was a surveillance plane which had flown out from the Cape Verde Islands, off the African coast, along the Airbus' flight path to its last known position.
Brazilian aircraft were involved in an overnight search and more are thought to have joined efforts in the morning. Spain and Senegal have also sent planes to help in the search.
Maria Celina Rodrigues, the Brazilian consul in Paris, said the depth of the ocean would make it difficult for searchers.
"They are hoping they can find debris, pieces, lifejackets that eventually float, but that takes some time," she told the Associated Press.
The Brazilian authorities have also said they are investigating a possible sighting of wreckage in the area, reported Reuters news agency.
The crew of a TAM Linhas Aereas flight travelling over the area in the other direction shortly after the Airbus' last signal had reported seeing "bright spots" in the ocean about 1,300 km (800 miles) from the Fernando de Noronha archipelago off Brazil's north-eastern coast.
'Unprecedented situation'
The plane's automatic reports were generated at around 0200 GMT on Monday, about four hours after Flight AF 447 left Rio de Janeiro, and as it was heading through turbulence towards the west African coast.
Missing man Arthur Coakleyâs wife, Patricia, and his business partner Ken Pearce
"A succession of a dozen technical messages" showed that "several electrical systems had broken down" which caused a "totally unprecedented situation in the plane", said Mr Gourgeon.
"It is probable that it was shortly after these messages that the impact in the Atlantic came," he told reporters at Charles de Gaulle airport, where the airliner had been due to land.
Flight AF 447 was flying at an altitude of 10,670m (35,000ft) shortly before it went missing.
A meteorologist who spoke to AP said tropical thunderstorms in the Atlantic could tower up to 15,000m (50,000ft).
French officials have stressed that the plane's captain was very experienced, clocking up more than 11,000 hours of flight.
Crisis centres have been set up at Charles de Gaulle airport in Paris and Rio's Tom Jobim international airport.
