Two suspect men who flew on a missing Malaysian airliner appear to have been Iranian illegal immigrants, officials said Tuesday, lessening fears of terrorism but shedding no light on the fate of the plane's 239 passengers and crew.

On the fourth day of a multinational search at sea and on land, relatives desperate for news of loved ones aboard Malaysia Airlines MH370 said their hopes for a miracle were ebbing away.

Authorities have doubled the search radius to 100 nautical miles (equivalent to 185 kilometres) around the point where the Boeing 777 disappeared from radar over the South China Sea early Saturday, en route from Kuala Lumpur to Beijing.

"We are intensifying our search and rescue, and hoping against hope there is still an opportunity for us to rescue (those on board)," Malaysian Transport Minister Hishammuddin Hussein told reporters.

The 34-year-old son of Malaysian security guard Subramaniam Gurusamy was on the flight to do business in Beijing for an oil company.

"My three-year-old grandson is asking: 'Where is Dad?' We tell him father has gone to buy sweets for you," Gurusamy, 60, said as he broke down in tears.

"Please bring back my son. I am praying for divine intervention. That is the only hope we have."

Malaysia had opened a terror probe, joined by FBI agents from the United States. But the revelation of the identities of two men who boarded the flight using stolen European passports suggested they were young Iranian migrants seeking a new life overseas.

Interpol named the pair as Pouria Nourmohammadi, 18, who was booked to fly on to Germany, and Seyed Mohammed Reza Delavar, 29, who was ticketed through to Denmark.

Delavar's ultimate destination was Sweden, where he intended to apply for political asylum, according to Swedish newspaper Aftonbladet.

The two travelled to Kuala Lumpur from Doha on their real Iranian passports, and their identification was helped by relatives in Europe who reported them missing, officials said.

"It is part of a human-smuggling issue and not a part of a terrorist issue," Interpol chief Ronald Noble told reporters in France, adding that the international agency was more and more "certain that these individuals are probably not terrorists."

However, the head of the US Central Intelligence Agency Tuesday said that terrorism could not be ruled out.

"Clearly this is still a mystery," said CIA Director John Brennan, stressing it was too early to draw any conclusions.

Iran has offered its assistance to the Malaysian investigation, pledging to provide "any information on the Iranians and their status as soon as it is available".

- People-smuggling ring? -

Police in Southeast Asia agreed that people-smuggling was emerging as the likeliest explanation for the identity fraud.

The two passports -- one Italian and one Austrian -- were stolen over the past two years in Thailand, where police have long been battling a thriving trade in Western documents used by criminal gangs.

"We believe that these two passports were stolen by a human-smuggling gang who send people to work in third countries, especially European countries," Lieutenant General Panya Maman, commander of Thailand's southern police region, told AFP.

Malaysia's national police chief Khalid Abu Bakar said his officers were not ruling anything out but were now focusing on theories including a hijacking, sabotage or psychological problems among passengers or crew.

Flight MH370, captained by a veteran pilot, had relayed no indications of distress, and weather at the time was said to be good.

The vastness of the search zone reflects authorities' bafflement over the plane's disappearance. The operation has grown to involve 42 ships and 35 aircraft from Southeast Asian countries, Australia, China, New Zealand and the United States.

The plane's last confirmed radar sighting was off Vietnam's southern coast. "In terms of our assessments and predictions -- we have little hope of a positive outcome," Pham Quy Tieu, Vietnamese deputy minister of transport, said.

- 'Emotional breakdown' -

The 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.

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

China, which had 153 of its nationals on board the plane, said it would harness 10 satellites equipped with high-resolution imaging to help in the search. Boeing said it was joining a US government team to try to unravel the mystery of what happened to its 777-200 plane.

Conflicting information has deepened the anguish of relatives, with tests on oil slicks in the South China Sea showing they were not from the missing jet and reports of possible debris from the flight also proving to be false alarms.

At a hotel in Beijing where relatives are gathered, a man in his 20s surnamed Su said: "I hope it is a hijacking, then there will be some hope that my young cousin has survived.

"My uncle and aunt had an emotional breakdown, they are not eating, drinking and sleeping."

A total of 17 Chinese relatives have so far taken up an offer from Malaysia Airlines to fly to Kuala Lumpur to be closer to the operation, and more are expected in the coming days, the airline said.

No idea where to look: the hunt for MH370
Phu Quoc, Vietnam (AFP) March 11, 2014 - Helicopters and planes criss-cross the sky as scores of boats search below -- but officials say the multi-national hunt for 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 Malaysia Airlines MH370 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 fourth day of searching, the operation had grown to involve 42 ships and 35 aircraft from Southeast Asian countries, 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.