KUALA LUMPUR (Reuters) - Officials investigating the disappearance of a Malaysian airliner with 239 people on board are narrowing the focus of their inquiries on the possibility that it disintegrated in mid-flight, a senior source said on Sunday.

Malaysia Airlines flight MH370 vanished after climbing to a cruising altitude of 35,000 feet between Kuala Lumpur and Beijing in the early hours of Saturday, but search teams have still not been able to make any confirmed discovery of wreckage in seas beneath the plane's flight path almost 48 hours after it took off.

"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 the 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.

The Malaysian authorities have said they are focused on finding the plane and have declined to comment when asked about the investigations.