Vietnam says its search planes have spotted oil slicks in the sea near where a Malaysia Airlines jet carrying 239 people mysteriously vanished on Saturday, in the first hint at the aircraft's possible fate.
Six Australians were on board the flight.
The announcement came more than 18 hours after flight MH370 slipped off radar screens somewhere between Malaysia's east coast and southern Vietnam, triggering an international search effort.
"Two of our aircraft sighted two oil slicks around 15 to 20km long, running parallel, around 500 metres apart from each other," the army's deputy chief-of-staff, Vo Van Tuan, told state-run VTV.
"We are not certain where these two oil slicks may have come from so we have sent Vietnamese ships to the area."
Air search operations stopped at nightfall, though ships continued searching, the airline said.
"I think the two oil slicks are very likely linked to the missing plane," Vice-Admiral Ngo Van Phat, who is helping to direct the search mission, told AFP.
"However, we have to check carefully once our rescue boats get access to the area," he said, adding the boats were expected to search a wide expanse of sea in darkness.
A senior Malaysia Airlines official told a press conference at a hotel near Beijing Capital International Airport early on Sunday that the "search and rescue team has yet to determine the whereabouts of MH370", 24 hours after it disappeared.
The official said Malaysian authorities had "not received any information if the oil slicks have anything to do with the missing plane".
The twin-engine jet had been flying from Kuala Lumpur to Beijing, where anguished relatives were still desperately waiting for news. It was carrying 227 passengers and 12 crew.
Adding to the mystery over the sudden loss of communication with the aircraft, it emerged that two passengers appeared to have been travelling on stolen EU passports.
An Austrian, named in reports as Christian Kozel, had his passport stolen in Thailand in 2012, while Italian Luigi Maraldi, 37, had his stolen last year, also in Thailand, officials and sources said.
Despite their names being on the passenger manifest, neither man was on the flight to Beijing.
Speaking to reporters in Malaysia, Deputy Transport Minister Datuk Aziz Kaprawi said authorities were probing the matter for possible foul play. "The information is still under review," he was quoted as saying by the Malay Mail Online.
In Washington, a US administration official said authorities were aware of the reporting of the two stolen passports.
"We have not determined a nexus to terrorism yet, although it's still very early and that's by no means definitive. We're still tracking the situation," the official said.
MH370 had relayed no distress signal, indications of rough weather, or other signs of trouble, and both Malaysia's national carrier and the Boeing 777-200 model used on the route are known for their solid safety records.
"We are looking at all possibilities but it is too soon to speculate," Malaysian Prime Minister Najib Razak said, when asked whether terrorism could have been a factor.
The pilot had flown for the carrier since 1981, Malaysia Airlines said. The plane was more than 11 years old.
