Vietnamese navy planes have spotted what could be fragments from the missing Malaysia Airlines jet that disappeared almost two days ago.
Officials said it was too dark to be certain the objects were from Flight MH370, which had 239 people on board.
A multinational team is searching for wreckage and ships will try to confirm the find after dawn.
Investigators are also checking CCTV footage of two passengers who were travelling on stolen passports.
Malaysian military officials said on Sunday that the plane may have turned back from its scheduled route shortly before vanishing from radar screens, further deepening the mystery surrounding its fate.
Relatives of the missing passengers have been told to prepare for the worst.
Flight MH730 left Kuala Lumpur, bound for Beijing, at 00:41 local time on Saturday (16:41 GMT on Friday). But radio contact was lost at 17:30 GMT, somewhere between Malaysia and Vietnam.
Late on Sunday, the Vietnamese authorities said possible debris from the plane had been spotted in the sea off south Vietnam.
"We received information from a Vietnamese plane saying that they found two broken objects, which seem like those of an aircraft, located about 50 miles to the south-west of Tho Chu Island," an unnamed official from the National Committee for Search and Rescue told AFP news agency.
"As it is night they cannot fish them out for proper identification. They have located the position of the areas and flown back to the land," he added.
The potential debris was in a similar area to a possible oil slick seen by Vietnamese navy planes on Saturday, but officials have cautioned that this too may be nothing to do with the disappearance of Flight MH370.
There are now 40 ships and 34 aircraft from nine different nations taking part in the search for the missing plane in the seas off Vietnam and Malaysia.
Other teams are investigating the identities of some of the people onboard.
Malaysia's civil aviation chief, Azharuddin Abdul Rahman, said five passengers booked on the flight did not board and their luggage was consequently removed.
It has also been confirmed that two passengers were travelling on stolen passports.
The passengers - travelling with Italian and Austrian passports that had been stolen in Thailand - purchased their plane tickets at the same time, and were both booked on the same onward flight from Beijing to Europe on Saturday.
Both had purchased their tickets from China Southern Airlines, which shared the flight with Malaysia Airlines, and they had consecutive ticket numbers.
"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 databases," the Secretary General of international police agency Interpol, Ronald Noble, said in a statement.
He said no checks of Interpol's database had been made for either passport between the time they were stolen and the departure of the flight, and expressed frustration that few of Interpol's 190 member countries "systematically" search the database.
Malaysia's Transport Minister Hishammuddin Hussein said all angles were being examined in the search for the what happened, but he added: "The main thing here for me and for the families concerned is that we find the aircraft."
The passengers on the flight were of 14 different nationalities. Two-thirds were from China, while others were from elsewhere in Asia, North America and Europe.
Malaysia Airlines is the country's national carrier, and one of Asia's largest fleets, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
Correspondents say the route between Kuala Lumpur and Beijing has become more and more popular as Malaysia and China increase trade.