Malaysian authorities have previously said flight MH370 disappeared about an hour after it took off from Kuala Lumpur for the Chinese capital Beijing.
At the time it was roughly midway between Malaysia's east coast town of Kota Bharu and the southern tip of Vietnam, flying at 35,000 ft (10,670 metres).
"It changed course after Kota Bharu and took a lower altitude. It made it into the Malacca Strait," the military official, who has been briefed on investigations, told Reuters.
The Strait of Malacca, one of the world's busiest shipping channels, runs along Malaysia's west coast.