The Malaysia Airlines flight missing with 239 on board may have turned around just before it vanished from radar screens, the country's air force chief said on Sunday as the government said it had contacted counter-terrorism agencies around the world following concerns over unidentified passengers.

Transport and defence minister Hishamuddin Hussein said officials were considering all possible explanations for the disappearance of flight MH370, adding: "We cannot jump the gun. Our focus now is to find the plane."

The airline warned families to prepare for the worst as the search widened amid inconclusive reports that debris had been spotted floating in the sea between Vietnam and Malaysia.

At least two people on the plane were travelling together on stolen passports, fuelling concerns about the Boeing-777's abrupt disappearance in the early hours of Saturday. However, experts said there were many possible reasons for why it vanished and for people to travel on false documents.

Malaysian officials said they were looking at four suspect identities and were examining the entire passenger manifest. Interpol confirmed that at least two passports were listed in its database as stolen and that it was examining other documents.

The international police agency's secretary general, Ronald Noble, said it had spent years urging countries to screen all passports systematically. "Now, we have a real case where the world is speculating whether the stolen passport holders were terrorists, while Interpol is asking why only a handful of countries worldwide are taking care to make sure that persons possessing stolen passports are not boarding international flights," he said.

Two-thirds of the travellers were Chinese, while the rest were from elsewhere in Asia, North America and Europe.

The vast search area in the seas between Malaysia and Vietnam expanded further on Sunday because of the plane's apparent turn off course. At least 40 ships and 22 aircraft from Vietnam, Malaysia, Thailand, Singapore, Indonesia, China and the United States are participating in the hunt.

The head of the MMEA, Mohd Amdan Kurish, left, checks a radar during the search for the missing plane. Photograph: AP

The director general of the Malaysian Maritime Enforcement Agency (MMEA) said it had sent a patrol ship to gather samples from an oil slick to determine whether the oil came from the flight or a passing ship, the government-backed Bernama website reported. No debris was found nearby.

The Beijing-bound Boeing-777 had reached cruising altitude when it disappeared from radar screens around 40 minutes after taking off from Kuala Lumpur at 12.40am on Saturday. Both the airline and the aircraft model have strong safety records. The weather was generally good and the plane did not issue a distress signal.

Malaysia's air force chief Rodzali Daud told a press conference that it appeared to have gone off-route. "We are trying to make sense of this … The military radar indicated that the aircraft may have made a turn back and in some parts, this was corroborated by civilian radar," he said.

The chief executive of Malaysia Airlines, Ahmad Jauhari Yahya, said the plane had not informed the airline and air traffic control authorities of its change of course, as it was supposed to do in such circumstances.

The pilot of another flight told a Malaysian newspaper he had made brief contact with the plane via his emergency frequency at the request of Vietnamese aviation authorities who were expecting it to enter their airspace but had been unable to reach it.

The unnamed man said he was deep into Vietnamese airspace when officials asked him to relay to MH370 to establish its position, and that he succeeded at around 1.30am – around 10 minutes after the last recorded flight data.

"There [was] a lot of interference … static … but I heard mumbling from the other end," he said, adding that he believed the voice belonged to the co-pilot, Fariq Abdul Hamid. He then lost the connection, he told the New Straits Times.

An airline pilot who flew within 100 nautical miles of the route 12 hours before the Malaysia Airline flight disappeared said there were large thunderstorms in the area, with some turbulence, but the weather did not appear to pose serious problems for commercial flights.

While the circumstances of the plane's disappearance are extremely unusual, an Air France flight vanished without warning over the South Atlantic in 2009; investigators blamed that crash on a combination of technical problems and pilot error.

Experts also cautioned that there were various reasons why people could be on board with invalid documents.

In 2010, when an Air India Express flight overshot the runway at Mangalore, killing 160, it emerged that 10 of those on board had fraudulent passports.

One US Department of Homeland Security official told the Los Angeles Times: "Just because they were stolen doesn't mean the travellers were terrorists."

Vietnamese air force crew prepare to head out from Tan Son Nhat airport to search for the Malaysian airliner. Photograph: AP

The passengers, using the names Luigi Maraldi and Christian Kozel, bought consecutive tickets on 6 March from China Southern, which had a codeshare agreement for the flight, paying in Thai baht. They were booked together to Beijing, where they would have had a stopover of just over 10 hours before travelling onwards to Amsterdam. "Maraldi" was then booked to fly to Copenhagen while "Kozel" was booked on a flight to Frankfurt.

Azharuddin Abdul Rahman, Malaysia's civil aviation chief, told a press conference investigators were looking at CCTV footage of the individuals on those passports "from check-in to departure".

The real Maraldi and Kozel – Italian and Austrian nationals respectively – had previously reported their passports stolen in the region. Maraldi told a Thai website he lost his in a deal that went wrong at a motorcycle rental shop in Phuket.

The Malaysian transport and defence minister Hishammuddin Hussein said authorities were looking at two more possible cases of suspicious identities. "All the four names are with me and have been given to our intelligence agencies," he said. "We are looking at all possibilities."

No information has been issued on the other cases under review. Chinese state media reported that one of the passport numbers reported on the manifest belonged to a man from Fujian, eastern China, who was safe and well. But his name was not that listed alongside the number, which according to the manifest belonged to another Chinese man. The man told police his passport had not been lost or stolen.

An FBI team is on its way to assist the investigation because three Americans were on board. Experts from the National Transportation Safety Board – which investigates all US domestic civil aviation crashes – the Federal Aviation Administration and Boeing are also flying to the region, the NTSB said in a statement.

The CEO of a Malaysia Airlines subsidiary told reporters the plane was last inspected 10 days ago and found to be "in proper condition".

Additional research by Cecily Huang