Based on new information, investigators of the missing Malaysia Airlines plane now believe "deliberate action" was behind what happened, said Malaysian Prime Minister Najib Razak at a press conference.
He said last confirmed communication between the plane and satellite was 8.11am, Malaysia time, on Saturday.
He also said two new areas of search were possible, after new data about the plane's last communication with the satellite.
One was a northern corridor, stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand.
The second was a corridor from Indonesia to southern Indian Ocean.
Mr Najib said the search in the South China Sea is being called off.
He said that based on new satellite information, investigators can say "with a high degree of certainty" that the Aircraft Communications Addressing and Reporting System (ACARS) was "disabled" just before the aircraft reached the east coast of peninsular Malaysia.
Shortly afterwards, near the border between Malaysian and Vietnamese air traffic control, the aircraft's transponder was "switched off".
From this point onwards, the Royal Malaysian Air Force primary radar showed that an aircraft which was believed - but not confirmed - to be MH370 did indeed turn back.
It then flew in a westerly direction back over peninsular Malaysia before turning northwest.
"Up until the point at which it left military primary radar coverage, these movements are consistent with deliberate action by someone on the plane," he said.
Today, based on raw satellite data that was obtained from the satellite data service provider, investigators can confirm that the aircraft shown in the primary radar data was flight MH370, he said.
Of the last contact at 8.11am, he said the team "is making further calculations which will indicate how far the aircraft may have flown after this last point of contact" to help refine the search.
"Due to the type of satellite data, we are unable to confirm the precise location of the plane when it last made contact with the satellite".
But the new data allowed investigators to determine that the plane's last communication with the satellite was in one of two possible corridors - a northern corridor stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand, or a southern corridor stretching approximately from Indonesia to the southern Indian ocean.
The investigation team is working to further refine the information.
He stressed that despite media reports that the plane was hijacked, "I wish to be very clear: we are still investigating all possibilities as to what caused MH370 to deviate from its original flight path".