The Vietnam Air Force has reportedly spotted two oil slicks in the sea, thought to be from a plane missing off the coast.
Search teams from several countries are looking for traces of a Malaysia Airlines Boeing 777.
The flight from Kuala Lumpur to Beijing was carrying 239 passengers and crew - although no Irish people were on board.
It disappeared about two hours into the flight.
The Malaysian Prime Minister Najib Razak is refusing to speculate about any terrorist involvement.
A Vietnamese government statement said they were consistent with the kind of spills caused by fuel from a crashed airliner.
Rescue boats are being sent to the area from the nearby island of Phu Quoc to look for survivors.
Earlier, the country's state media quoted a navy official as saying Flight MH370 crashed near Tho Chu Islands at around 2.40am local time.
The US Navy is helping teams from Vietnam, Malaysia and Singapore search the vast waters in the Gulf of Thailand, between Malaysia and Vietnam.