Malaysia's civil aviation chief has said the fate of a missing Malaysia Airlines jet remains "a mystery".
Azharuddin Abdul Rahman said officials had not ruled out hijacking as a cause of the plane's disappearance.
He said all reported sightings of debris from the plane in the seas south of Vietnam were unconfirmed.
Malaysia Airlines flight MH370 vanished from radar almost three days ago en route to Beijing from Kuala Lumpur, with 239 people on board.
Relatives of the missing passengers have been told to prepare for the worst.
'Searching every second'
Azharuddin Abdul Rahman, who heads the department of civil aviation, told reporters in Kuala Lumpur: "Unfortunately we have not found anything that appears to be objects from the aircraft, let alone the aircraft."
He added that they were intensifying efforts to find the plane, and the search would "take as long as it takes".
"We are every hour, every second looking at every area of the sea."
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.
The debris was in a similar area to a possible oil slick seen by Vietnamese navy planes on Saturday.
But Azharuddin Abdul Rahman said on Monday that this sighting had never been officially verified and could not be confirmed.
Samples of the oil had been collected and sent to a laboratory for analysis to see if it came from the plane, he said.
US Navy officials earlier said their aircraft had not seen any debris associated with commercial aircraft wreckage.
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Flight MH370 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.
Officials say they still have no idea what went wrong.
Malaysian military officials said on Sunday they were widening the search area because of indications the plane, a Boeing 777-200ER, may have turned back from its scheduled route shortly before vanishing from radar screens.
Commander William Marks from the US Seventh Fleet, which is taking part in the search, told the BBC the search area covered 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."
Investigators are looking at all angles, including a possible terror attack. Counter-terrorism agencies and the FBI are involved in the operation.
The focus has been on two passengers who boarded the flight using stolen passports.
International police agency Interpol has confirmed the passengers were travelling with Italian and Austrian passports stolen in Thailand years ago.
They had purchased their tickets at the same time from China Southern Airlines which shared the flight with Malaysia Airlines, 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".
Five passengers booked on the flight did not board, and their luggage was consequently removed.
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.
Meanwhile, Malaysian King Abdul Halim Mu'adzam Shah offered his condolences to the passengers.
He said he and the Queen "would like to express our sympathy to the passengers and crew, especially to their family".
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% to a record low.