Vietnamese state media, quoting a senior naval official, had reported that the Boeing 777-200ER flight from Kuala Lumpur to Beijing had crashed off south Vietnam, but Malaysia's transport minister later denied any crash scene had been identified.
"We are doing everything in our power to locate the plane. We are doing everything we can to ensure every possible angle has been addressed," Transport Minister Hishamuddin Hussein told reporters near the Kuala Lumpur International Airport.
"We are looking for accurate information from the Malaysian military. They are waiting for information from the Vietnamese side," he said.
Vietnam's state-run Tuoi Tre news also quoted Admiral Ngo Van Phat as qualifying his earlier remarks about a crash site having been identified, saying he had been referring to a presumed crash site beneath the plane's flight path using information supplied by Malaysia.
A crash, if confirmed, would mark the U.S.-built Boeing 777-200ER airliner's deadliest incident since entering service 19 years ago.
Malaysia Airlines plane disappeared without giving a distress signal - a chilling echo of an Air France flight that crashed into the South Atlantic on June 1, 2009, killing all 228 people on board. It vanished for hours without issuing a distress call.
VANISHED AFTER REACHING 35,000 FEET
Malaysia Airlines Flight MH370, operating a Boeing 777-200ER aircraft, last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, Malaysia Airlines chief executive Ahmad Jauhari Yahya said in a statement read to an earlier news conference in Kuala Lumpur.
Flight tracking website flightaware.com showed the plane flew northeast over Malaysia after takeoff and climbed to an altitude of 35,000 feet. The flight vanished from the website's tracking records a minute later while it was still climbing.
Malaysia and Vietnam were conducting a joint search and rescue, he said but gave no details. China and the Philippines have sent ships to the South China Sea to help in any search and rescue.
"We are extremely worried," Chinese Foreign Minister Wang Yi told reporters in Beijing before the initial Vietnamese report that the plane had crashed. "The news is very disturbing. We hope everyone on the plane is safe."
The Malaysia Airlines flight left Kuala Lumpur at 12.21 a.m. (1621 GMT Friday) but no trace had been found of the plane more than eight hours after it was due to land in the Chinese capital at 6.30 a.m. (2230 GMT Friday) the same day.
"We deeply regret that we have lost all contacts with flight MH370," Jauhari said.
The Malaysia Airlines said people from at least 14 nationalities were among the 227 passengers - at least 152 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans. Two infants were on board.
If it is confirmed that the plane has crashed, the loss would mark the second fatal accident involving a Boeing 777 in less than a year and by far the worst since the jet entered service in 1995.
An Asiana Airlines Boeing 777-200ER crash-landed in San Francisco in July 2013, killing three passengers and injuring more than 180.
Boeing said it was aware of reports that the Malaysia Airlines plane was missing and was monitoring the situation but had no further comment. The flight was operating as a China Southern Airlines codeshare.
An official at the Civil Aviation Authority of Vietnam (CAAV) said the plane had failed to check in as scheduled at 1721 GMT while it was flying over the sea between Malaysia and Ho Chi Minh city.