The head of Malaysia's Civil Aviation Authority says the fate of a missing Malaysia Airlines jet is "a mystery", reports BBC.
Azharuddin Abdul Rahman said officials had not ruled out hijacking as a cause of the plane's disappearance, BBC reported.
He said all reported sightings of debris from the plane in the seas south of Vietnam were unconfirmed.
Malaysia Airlines flight MH370 vanished almost two days ago en route to Beijing from Kuala Lumpur, with 239 people on board.
Relatives of the missing passengers have been told by the authorities to prepare for the worst.
The Civil Aviation chief told reporters in Kuala Lumpur: "We are every hour, every second looking at every area of the sea."
Malaysia Airlines lost contact with Flight MH370 for five hours before it confirmed the news.
The slow pace of information forced Malaysians to turn to social media first – then ask the authorities to confirm speculation or reports that appeared online.
Among the many questions was how two passengers with fake European passports could have boarded the flight.
Over the past four years, I have travelled frequently through the same airport.
As a Canadian passport holder I have to scan both index fingers before I enter the country but not when I leave.
The biometric system was set up in 2011 to prevent foreigners from repeatedly coming in to work illegally and to curb human trafficking and wildlife smuggling.
Malaysian officials say they are working hard to answer questions. They have reminded people to avoid speculation, but it hasn't reassured distressed family members.
WHAT WE KNOW ABOUT FLIGHT MH370
An investigation into the cause of the plane's disappearance is focusing on two passengers who boarded the flight using stolen passports.
Malaysian military officials said on Sunday that the plane, a Boeing 777-200ER, may have turned back from its scheduled route shortly before vanishing from radar screens, further deepening the mystery surrounding its fate.
CONTACT LOST
Flight MH730 left Kuala Lumpur for Beijing at 00:41 local time on Saturday (16:41 GMT on Friday). But radio contact was lost at 17:30 GMT, somewhere between Malaysia and Vietnam.
There are now 40 ships and 34 aircraft from nine different nations taking part in the search in the seas off Vietnam and Malaysia.
Late on Sunday, the Vietnamese authorities said two objects had been sighted in waters south of Vietnam which appeared to resemble aircraft parts, including a door.
It was in a similar area to a possible oil slick seen by Vietnamese navy planes on Saturday.
But the Malaysian authorities said on Monday that this sighting had never been officially verified and could not be confirmed.
US Navy officials earlier said their aircraft had not seen any debris associated with commercial aircraft wreckage.
Commander William Marks from the US Seventh Fleet, which is taking part in the search, told the BBC the search was covering hundreds of kilometres.
"Just from the air we can see things as small as almost the size of your hand, or a basketball. So it's not a matter of if we can see it – it's an extremely large area.
Officials say they still have no idea what happened to the aircraft.
Investigators are looking at all angles, including a possible terror attack. Counter-terrorism agencies and the FBI are involved in the operation.
The identities of some of the people onboard are being probed.
International police agency Interpol has confirmed that two passengers were travelling on passports registered on its databases as stolen.
The passengers – travelling with Italian and Austrian passports stolen in Thailand years ago – had purchased their tickets at the same time from China Southern Airlines, which shared the flight with Malaysia Airlines.
They had consecutive ticket numbers and were both booked on the same onward flight from Beijing to Europe on Saturday.
Ronald Noble, the secretary general of Interpol, said in a statement that while it was too soon to speculate on any connection between the theft and the plane's disappearance, it was "clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol databases".
He said no checks of Interpol's database had been made for either passport between the time they were stolen and the departure of the flight, and expressed frustration that few of Interpol's 190 member countries "systematically" search the database.
Five passengers booked on the flight did not board, and their luggage was consequently removed.
WHO WERE PASSENGERS?
The passengers on the flight were of 14 different nationalities. Two-thirds were from China, while others were from elsewhere in Asia, North America and Europe.
China has sent a team of government officials to Kuala Lumpur to look into the case.
The BBC's John Sudworth in Beijing says relatives waiting for news are getting increasingly frustrated, saying they are not getting information in a timely manner.
They have complained that government officials have not visited the families, our correspondent adds.
Malaysia Airlines is the country's national carrier, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
On Monday, shares in Malaysia Airlines fell 18 percent to a record low.
BBN/ANS/AD/10March14/11:40 am (BST)
