The fate of the missing Malaysian Airlines jet believed to have disappeared off the coast of Vietnam almost three days ago with 239 people aboard remains a mystery, officials said.

Malaysia's civil aviation head, Azharuddin Abdul Rahman, speaking to reporters in Kuala Lumpur Monday, said that investigators have so far not found anything that could be part of the missing plane.

But that statement came as Vietnam sent helicopters to investigate a floating "yellow object" suspected of being a life raft from flight MH370 bound for Beijing, Reuters reported. Vietnam's civil aviation body later ruled that out, saying the object was not from the plane.

Separately, six planes and seven ships from Vietnam have so far been unable to find an object spotted by a low-flying plane on Sunday afternoon, authorities said.

Doan Huu Gia, the chief of Vietnam's search and rescue coordination center, said the search for the rectangular piece of debris has proved unsuccessful.

Vietnamese officials said they believe the object is one of the plane's doors, according to local news media reports.

Vietnam civil administration chief Pham Viet Dung said search teams from several countries sent boats to the area about 56 miles south of Tho Chu island, in an area where an oil slick was spotted Saturday. Malaysian maritime officials found some oil slicks in the South China Sea and sent a sample to a lab to see if it came from the plane, Rahman told a news conference on Monday.

Authorities said earlier that they had spotted an object in the area that turned out not to be from the plane.

Flight MH370 vanished early Saturday two hours into a scheduled six-hour flight from Kuala Lumpur. A Malaysian official said Sunday the plane may have tried to return before disappearing.

Air force chief Rodzali Daud said military radar indicated the flight "may have made a turn back," but he did not say how far it got. "We are trying to make sense of this," Daud said.

Dozens of ships and aircraft have failed to find any piece of the missing Boeing 777 as investigators pursue "every angle" to explain its disappearance, including hijacking, Malaysia's civil aviation chief said Monday.

The U.S. Navy has provided the USS Pinckney, a guided-missile destroyer that carries two MH-60R helicopters, and a P-3C Orion with long-range search, radar and communications capabilities.

Reuters, citing what it called a senior source involved in the investigation, said the probe is focusing on the possibility that the plane disintegrated in the air.

"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.

Malaysia Airlines has been telling relatives "to expect the worst," spokesman Ignatius Ong said.

Hundreds of distraught relatives gathered in a hotel in Beijing on Monday, waiting to be flown to Malaysia. Of the 227 passengers, two-thirds were Chinese. There were also 38 passengers and 12 crew members from Malaysia, and others from elsewhere in Asia, Europe and North America, including three Americans.

An international team is investigating the crash. American experts include accident investigators from the National Transportation Safety Board and technical experts from the Federal Aviation Administration and Boeing, the NTSB said in a statement.

Authorities are investigating the possibility of terrorism after discovering that two passengers apparently had been flying with stolen passports. Interpol, the France-based international policing agency, confirmed Sunday that the Italian and Austrian passports had been entered into its database after they were reported stolen in 2012 and 2013.

"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol's databases," said Interpol Secretary-General Ronald Noble.


Contributing: Associated Press