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.
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.