Malaysia's military believes it tracked the missing plane on radar over the Strait of Malacca, some distance away from where it last made contact with air traffic control.
"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 runs along the west coast of Malaysia and is one of the world's busiest shipping channels.
The airline said on Saturday that all contact was lost off the eastern coast town of Kota Bharu.