A senior rescue official says Malaysian naval vessels saw no immediate sign of wreckage when they reached the maritime area off the country's northeast coast where a missing Malaysia Airlines flight last made contact.
Malaysia has been joined by China, Vietnam and the Philippines in searching the the plane which had 239 people, including two New Zealanders, on board.
Malaysia Airlines says that in addition to the two New Zealanders, there were six Australians on board its Boeing 777-200 when it crashed into the sea off Vietnam.
A Vietnam Navy official says the passenger jet was flying between Kuala Lumpur and Beijing when it crashed into the sea near Vietnam's Tho Chu Island, while flying between Kuala Lumpur and Beijing .
There has been no confirmation of the claim.
"At the moment there are no Vietnamese navy boats in that area so we have to ask boats from Phu Quoc island to be prepared for rescue," Admiral Ngo Van Phat told the website of Tuoi Tre news.
Tho Chu and Phu Quoc lie to the southwest of southern Vietnam.
China's Xinhua news agency said earlier the Boeing Triple Seven aircraft never entered Chinese airspace and may have disappeared while flying over Vietnam.
It is carrying 227 passengers, including two children, and 12 crew members.
The passengers included 13 other nationalities, in addition to the New Zealanders: 153 Chinese citizens, 38 Malaysians, 7 Indonesians, 5 Indians, 6 Australians, 3 French, 4 Americans, 2 Ukrainians, 2 Canadians, 1 Russian, 1 Italian, 1 Taiwanese, 1 Dutch and 1 Austrian.
Late on Saturday, New Zealand's Ministry of Foreign Affairs said the next of kin of the two New Zealanders on the passenger list had yet to be informed. But it's Australian counterpart said it had confirmed the names of six Australians on the flight, and Australian media named two Queensland couples from Brisbane and Ipswich.
The missing people include two infants.
The airline says Flight MH370 disappeared about 7:30am New Zealand time. It had been due to land in Beijing at 11.30am New Zealand time.
"Malaysia Airlines is currently working with the authorities who have activated their search and rescue team to locate the aircraft," the carrier said.
"Our team is currently calling the next-of-kin of passengers and crew."
The vice president of the Airlines' operations, Fuad Sharuji, said 153 of the passengers were Chinese, and the passengers included 14 different nationalities.
"We deeply regret that we have lost all contacts with flight MH370 which departed Kuala Lumpur at 12.41 am earlier this morning bound for Beijing," said the airline.
The flight went missing two hours after taking off from Kuala Lumpur.
China's Xinhua state news agency said it was lost in airspace controlled by Vietnam.
The aircraft did not enter airspace controlled by China and did not make contact with Chinese controllers, the agency added.
"Our thoughts and prayers are with all affected passengers and crew and their family members," Malaysia Airlines said its statement.
The airline is the national carrier of Malaysia and one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations world-wide.