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, reports BBC.
Malaysia has also said it was sending ships to investigate a "yellow object" spotted floating in Vietnamese waters.
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.
At a news conference on Monday, Malaysia's armed forces chief also said they were co-operating with Chinese intelligence to identify the two passengers on board flight MH370 who were travelling on stolen passports.