Officials say two passengers on the missing Malaysia Airlines jet were traveling with stolen passports.
Two men listed on the flight's manifesto - one from Italy and another from Austria - never boarded the flight from Kuala Lumpur to Beijing that disappeared early Saturday carrying 239 people. Both had their passports stolen in Thailand within the last two years.
The Austrian is in his home country and the Italian is still living in Thailand. The true identity of those traveling on the plane with the stolen documents remains unclear.
Meanwhile the Vietnamese government is sending rescue boats where search planes spotted two large oil slicks.
The slicks, each about 15 kilometers long, were found off the southern tip of the country near in an area where the aircraft vanished.
The Pentagon has dispatched a naval destroyer and a surveillance plane to aid in the search.
Ships and aircraft from Malaysia, Vietnam, China and the Philippines have concentrated their search in an area about 240 kilometers off the coast of Vietnam's southwestern Tho Chu island. Vietnamese authorities say that is where they last detected a signal from the Boeing 777-200.
Malaysia Airlines says contact was lost with Flight MH370 about one hour after it took off from the Malaysian capital.
At a news conference Saturday, Malaysia Airlines CEO Ahmed Jauhari Yahya said the airline is working with search and rescue teams to locate the aircraft.
The company's Facebook page said the passengers are from 14 nations, including 152 Chinese and 38 Malaysians. State Department spokeswoman Jen Psaki said three Americans were on board the flight.
China's official Xinhua news agency reported contact with the plane was lost in Vietnamese airspace. It said the plane never entered China's air traffic control area. Vietnamese officials said the flight disappeared about a minute short of entering Vietnamese airspace.
The most recent accident involving a Boeing 777 was the Asiana Airlines crash at San Francisco International Airport in July, 2013, in which three people died. Pilot error is suspected in that incident.