A Vietnamese navy plane has spotted an object suspected of belonging to a Malaysian jetliner that went missing early on Saturday with 239 people on board, the Civil Aviation Authority of Vietnam said on its website on Sunday.

The authority said it was too dark to be certain the object was part of the missing plane, and that more aircraft would be dispatched to investigate the site, in waters off southern Vietnam, in the morning.

Malaysian officials had earlier said no wreckage had yet been found, despite a search involving 34 aircraft and 40 ships.

Meanwhile, officials investigating the disappearance of the airliner are narrowing the focus of their inquiries on the possibility that it disintegrated in mid-flight, a senior source said.

Malaysia Airlines flight 370 vanished after climbing to a cruising altitude of 35,000 feet between Kuala Lumpur and Beijing in the early hours of Saturday.

"The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet," said the source, who is involved in the investigations in Malaysia.

If the plane had plunged intact from such a height, breaking up only on impact with the water, search teams would have expected to find a fairly concentrated pattern of debris, said the source, speaking on condition of anonymity because he was not authorised to speak publicly on the investigation.

The source was speaking shortly before Vietnamese authorities said a military plane had spotted at sea an object suspected to be part of the missing airliner.

Asked about the possibility of an explosion, such as a bomb, the source said there was no evidence yet of foul play and that the aircraft could have broken up due to mechanical issues. - Reuters