The Boeing 777-200ER flight from Kuala Lumpur to Beijing had been missing for hours when Vietnam's Tuoi Tre news quoted Admiral Ngo Van Phat as saying he had asked boats from an island off south Vietnam to rush to the crash site.
Malaysia Airlines had yet to confirm that the aircraft had crashed. It said earlier in the day that no distress signal had been given and cited early speculation that the plane may have landed in Nanming in southern China.
If the report that the plane crashed is confirmed, it would mark the US-built airliner's deadliest crash since entering service 19 years ago. 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.