Rescue efforts are ongoing to find EgyptAir flight MS804 which went missing from Paris to Cairo with 66 passengers and crew on board.
Egypt's Prime Minister Sherif Ismail said the search was under way to find the plane and it was too early to rule out any explanation for the incident, including terrorism.
"We cannot exclude anything at this time or confirm anything. All the search operations must be concluded so we can know the cause," he told reporters at Cairo airport.
Aviation editor at West Australian Newspapers, Geoffrey Thomas, believes an explosion onboard the aircraft led to its disappearance over the Mediterranean Sea, some 280km from the Egyptian coast.
Egyptian military search teams confirmed to receiving a distress call from the emergency mechanisms of the missing plane at 4.26am (1226 AEST), almost two hours after it disappeared from radar, the airline says.
"It’s too early to tell what might be the problem here, but that it disappeared at altitude indicates that something catastrophic has happened," he said.
"This plane just vanished. For a plane to vanish off radar, means something catastrophic, something pretty awful. Usually something explosive.
"I would probably rule out mechanical, because if it was a mechanical issue, there would be possibly warnings from the captain or we would still be tracking it as it descended."
Relatives grieve as they wait in front of EgyptAir in-flight service building where they been being held at Cairo International Airport Source: AAP
'Wreckage should be found soon'
Head of the School of Aviation at the University of NSW, Professor Jason Middleton, said he would be "surprised" if plane wreckage wasn't found within hours.
"That part of the eastern part of the Mediterranean is choc-full of US carriers and other ships involved in the Syrian conflict. It's a very busy part of the world from a civilian plane context," he said.
"Unlike the MH370 which disappeared in the Southern Ocean where nobody ever goes, I wouldn't be surprised if wreckage is found very quickly."
Prof Middleton ruled out any possibility that the plane landed safely.
"It's a jet plane that needs a long run way," he said.
"That is stretching the bounds of possibility, I think it's unlikely the plane has landed safely or has been hijacked and landed safely."
Mr Thomas said staff at Paris' Charles De Gualle Airport could also have played a role in the disappearance.
"I would’ve thought that Paris Airport security would be at a high level, the big danger we have in the industry is what we call the inside job, where airport employees could possibly be part of the problem," he said.
Mr Thomas said the airline had "turned the corner" in improving a "checkered record" of flight security.
Calls for caution
Will Geddes, managing director of private security firm International Corporate Protection, urged people to be "cautious" about assuming the disappearance was caused by terrorism because the flight had departed from Paris, which has suffered several attacks in recent years.
"There's a lot of concentrated focus on terrorist issues around Paris but one has to be cautious about drawing conclusions," he said.
But Geddes added that even if it is confirmed the plane did give an emergency signal, that would not rule out terrorism as a possibility because an explosion would not necessarily have caused the immediate destruction of the plane.
"It would not have needed to be a large device - if it was a terrorist-related failure - to lose cabin pressure," he said.
-With AAP
