Helicopters and planes criss-crossed the sky as scores of boats search below — but officials say the multi-national hunt for the missing flight MH370 is like looking for a needle in a haystack.

Malaysia on Monday doubled the search radius to 100 nautical miles (equivalent to 185 kilometres) around the point where the Malaysia Airlines (MAS) aircaft disappeared from radar over the South China Sea early Saturday.

“The biggest problem is just knowing where to look — especially at night,” Vo Van Tuan, a top Vietnamese military officer who is leading Vietnam’s search effort, told AFP.

The vastness of the search zone reflects authorities’ bafflement over the plane’s disappearance.

On the fifth day of the search mission, the operation had grown to involve 42 ships and 35 aircraft from Southeast Asian countries, India, Australia, China, New Zealand and the United States. Japan said Tuesday it was sending a plane to join the search efforts.

Vietnam has mobilised its first major search and rescue operation, deploying aircraft, boats and its commercial fishing fleet to help Malaysia search for the jet — even as relatives of the 239 people aboard said their hopes for a miracle were ebbing away.

The hunt to discover the plane’s fate will likely be “a long mission that requires patience,” Vietnamese Major General Do Minh Tuan told AFP as he flew on a military helicopter near the country’s southern Tho Chu island.

“If the plane crashed and sank, some debris will surface, and if we find that we will be able to pinpoint the location of the plane,” he said.

But multiple reports of “suspicious floating objects” have revealed nothing but flotsam, tired Vietnamese rescue officials, putting in 20-hour days, concede.

“In terms of our assessments and predictions - we have little hope of a positive outcome,” Pham Quy Tieu, deputy minister of transport, said Tuesday.

In southern Phu Quoc, normally a sleepy tourist town, hundreds of foreign journalists — who usually face strict visa restrictions — have arrived after the government set up a search and rescue base at the airport.

Officials have taken over rooms in the air traffic control tower at the new Phu Quoc international airport, where the atmosphere is calm and organised, but sparsely furnished rooms hint at Vietnam’s limited resources.

The communist country “has minimal capabilities for search and rescue at sea,” said Vietnam expert Carl Thayer, adding it was geared more towards dealing with natural disasters such as typhoons.

“The longer the search continues (Vietnam) will have problems sustaining its commitment,” he said.

The total search sphere now includes land on the Malaysian peninsula itself, the waters off its west coast, and an area to the north of the Indonesian island of Sumatra. Vietnam has said it will search on land if needed.

That covers an area far removed from the scheduled route of MAS flight MH370, which officials say may have inexplicably turned back towards Kuala Lumpur.