Malaysia on Sunday said a missing airliner carrying 239 people may have inexplicably turned back, as authorities launched a terror probe into the plane's sudden disappearance, investigating suspect passengers who boarded with stolen passports.

The United States sent the FBI to investigate after Malaysia Airlines flight MH370 from Kuala Lumpur to Beijing vanished from radar early Saturday somewhere at sea between Malaysia and Vietnam, but stressed there was no evidence of terrorism yet.

Indications that the plane may have deviated from its route only compounded the anxiety of relatives, many of them Chinese, desperate for news of their loved ones.

"There is a distinct possibility the airplane did a turn-back, deviating from the course," said Malaysia's air force chief, General Rodzali Daud, citing radar data.

But Malaysia Airlines (MAS) chief executive Ahmad Jauhari Yahya said the Boeing 777's systems would have set off alarm bells.

"When there is an air turn-back the pilot would be unable to proceed as planned," he said, adding authorities were "quite puzzled" over the situation.

Malaysian authorities have expanded their search for wreckage to the country's west coast after initially concentrating to the east in the South China Sea.

A total of 40 ships and 34 aircraft from an array of Southeast Asian countries, China and the US were involved in the hunt, with two Australian surveillance aircraft due to join in.

An AFP photographer on board a Vietnamese military aircraft saw what appeared to be oil slicks in the sea off the country's coast and dozens of ships searching the area.

Some of the ships could be seen sailing parallel to each other in an apparently coordinated search. It was not clear which countries the ships belonged to.

However, as darkness fell on day two of the international effort, there were no confirmed reports of any wreckage being found.

After it emerged that two people boarded the flight with stolen European passports, Malaysia's transport minister Hishammuddin Hussein said he was looking at four suspect passengers in all.

He said authorities were examining CCTV footage of the two with fake passports.

"We have managed to get visuals of them," he said, adding that Malaysia was liaising with other countries' intelligence agencies on the findings. He gave no more details.

He also confirmed the US Federal Bureau of Investigation (FBI) was dispatching personnel to Malaysia.

"At the same time our own intelligence has been activated, and of course, the counter-terrorism units... from all the relevant countries have been informed," Hishammuddin said, refusing to rule out the possibility of a hijack.

A Malaysian civil aviation official said authorities still so far believe only two passengers had used stolen passports.

Prime Minister Najib Razak was quoted by The Star newspaper saying the government would review and enhance airport security protocols, if needed.

"If necessary, because we still do not know the cause of the incident," he was quoted as saying.

Flight MH370 had relayed no indications of distress, and weather at the time was said to be stable. Malaysia's national carrier and the Boeing 777-200 model used on the route are known for their solid safety records.

Technical advisers from Boeing and the US Federal Aviation Administration are en route to Asia to help with the probe.

In a statement, Boeing offered "its deepest concern to the families" of the missing.

The flight vanished about an hour after taking off from Kuala Lumpur. A total of 153 Chinese nationals were on board, and relatives camping out at Beijing's main international airport bemoaned the lack of news Sunday.

"The airline company didn't contact me, it was a friend," a middle-aged woman surnamed Nan told reporters, holding back tears. Her brother-in-law was on the flight.

"I can't understand the airline company. They should have contacted the families first thing."

Malaysia Airlines (MAS) insisted it was doing its best to keep relatives in China informed given the confusion over the plane's fate, and was preparing to fly some of them to Malaysia on Monday to be closer to the search-and-rescue operation.

Vietnamese Rear Admiral Ngo Van Phat told AFP he had "no information" on an assertion by another Vietnamese officer that boats had been sent to investigate an object floating in the South China Sea.

Two European names -- Christian Kozel, an Austrian, and Luigi Maraldi of Italy -- were listed on the passenger manifest but neither man boarded the plane, officials said. Both had their passports stolen in Thailand over the past two years.

China Southern Airlines, which has a code-share agreement with MAS, listed one Italian and one Austrian among its ticketed passengers on the flight. There were no other passengers of those nationalities listed on the MAS manifest.

Meanwhile, a Chinese national whose passport number was listed on the manifest did not board the plane and was still in China, the country's state media reported, adding that the actual passport was never stolen.

In Washington, a US administration official said authorities were aware of the passports issue but had "not determined a nexus to terrorism yet".

The flight carried 227 passengers and 12 crew. A US semiconductor company based in Texas said 20 of its employees were on the flight -- a dozen from Malaysia and eight from China.