The location is far from where it last made contact with air traffic control, off the country's east coast.
The Strait of Malacca, one of the world's busiest shipping channels, runs along Malaysia's west coast. The airline said on Saturday that the flight carrying 227 passengers and 12 crew last had contact off the east coast Malaysian town of Kota Bharu.
"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, said.
Meanwhile, a man from North-West Malaysia said he spotted a 'bright light descending into the Gulf of Thailand on Saturday night, leading searchers to doubt if the plane crashed into the sea.
The Beijing-bound Boeing 777-200 Flight MH370 of Malaysia Airlines had 239 passengers aboard, including five Indians and one Indian-origin Canadian, and 12 crew members. It went missing on Saturday morning.