The missing Malaysian airliner was deliberately diverted and continued flying for more than six hours after losing contact with the ground, Malaysia's leader said.
The plane could have gone as far north west as Kazakhstan or into the Indian Ocean's southern reaches.
Prime Minister Najib Razak's statement confirmed days of mounting speculation that the disappearance of Malaysia Airlines Flight MH370 to Beijing more than a week ago was not accidental.
It refocused the investigation into the flight's crew and passengers and underlined the massive task for searchers who already have been scouring vast areas of ocean.
"Clearly the search for MH370 has entered a new phase," Mr Najib said at a televised news conference.
He stressed that investigators were looking into all possibilities as to why the Boeing 777 deviated so drastically from its original flight path, saying authorities could not confirm whether it was a hijacking. Earlier, a Malaysian official said the plane had been hijacked, though he added that no motive had been established and no demands had been made known.
"In view of this latest development, the Malaysian authorities have refocused their investigation into the crew and passengers on board," Mr Najib told reporters, reading from a written statement but not taking any questions.
Police today drove into the residential compound where the missing plane's pilot lives in Kuala Lumpur, according to a guard and several local reporters who were barred from entering the complex. Authorities have said they will investigate the pilots as part of their probe, but have released no information about how they are progressing.
Experts have previously said that whoever disabled the plane's communication systems and then flew the jet must have had a high degree of technical knowledge and flying experience. One possibility they have raised was that one of the pilots wanted to commit suicide.
The plane was carrying 239 people when it departed for an overnight flight from Kuala Lumpur to Beijing at 12.40am on March 8. Its communications with civilian air controllers were severed at about 1.20am, and the jet went missing, heralding one of the most puzzling mysteries in modern aviation history.
Investigators now have a high degree of certainty that one of the plane's communications systems - the Aircraft and Communications Addressing and Reporting System - was disabled before the aircraft reached the east coast of Malaysia, Mr Najib said. Shortly afterward, someone on board then switched off the aircraft's transponder, which communicates with civilian air traffic controllers.
Mr Najib then confirmed that Malaysian air force defence radar picked up traces of the plane turning back westward, crossing over Peninsular Malaysia into the northern stretches of the Strait of Malacca. Authorities previously had said this radar data could not be verified.
"These movements are consistent with deliberate action by someone on the plane," Mr Najib said.
Although the aircraft was flying virtually blind to air traffic controllers at this point, onboard equipment continued to send pings to satellites.
The prime minister said the last confirmed signal between the plane and a satellite came at 8.11am - 7 hours and 31 minutes after take-off. This was more than five hours later than the previous time given by Malaysian authorities as the possible last contact.
Airline officials have said the plane had enough fuel to fly for up to about eight hours.
"The investigations team is making further calculations which will indicate how far the aircraft may have flown after this last point of contact," Mr Najib said.
He said authorities had determined that the plane's last communication with a satellite was in one of two possible "corridors" - a northern one from northern Thailand through to the border of the Central Asian countries Kazakhstan and Turkmenistan, and a southern one from Indonesia to the southern Indian Ocean.
Searching in the South China Sea, where the plane first lost contact, has ended, Mr Najib said.
Two-thirds of the plane's 227 passengers were Chinese, and China's government has been under pressure to give relatives firm news of the plane's fate.
In a stinging commentary, the Chinese government's Xinhua News Agency accused Malaysia of dragging its feet in releasing information. Information released by the Malaysian leader is "painfully belated," the commentary said. It said delays had resulted in wasted efforts and strained the nerves of relatives.
"Given today's technology, the delay smacks of either dereliction of duty or reluctance to share information in a full and timely manner," Xinhua said. "That would be intolerable."
Chinese Foreign Ministry spokesman Qin Gang said China had urged Malaysia to release more details about the new search area.
The northern route described by Mr Najib might have taken the plane through a region home to extremist Islamist groups and unstable governments, as well as remote, sparsely populated areas. But the region also hosts US military bases with powerful surveillance capabilities.
Flying south would put the plane over the Indian Ocean, with an average depth of 12,762 feet and thousands of miles from the nearest land mass.
Malaysia has faced accusations that it is not sharing all its information or suspicions about the plane's final movements, which have been the subject of constant media leaks both in Malaysia and the United States. Mr Najib said that he understood the need for families to receive information, but that his government wanted to release only fully corroborated information.
He said that from day one, the country had been sharing information with international investigators, even when it meant placing "national security concerns" second to the search, a likely reference to its release of military radar data. US, British and Malaysian air safety investigators have been on the ground in Malaysia to assist with the investigation.
In the Chinese capital, relatives of passengers who have anxiously awaited news at a hotel near Beijing's airport said they felt deceived at not being told earlier about the plane's last signal. "We are going through a rollercoaster, and we feel helpless and powerless," said a woman, who declined to give her name.
At least one of the people waiting at the hotel saw a glimmer of hope in word that the plane's disappearance was a deliberate act, rather than a crash. "It's very good," said the woman, who gave only her surname, Wen.
Malaysian police have already said they are looking at the psychological state, family life and connections of pilot Zaharie Ahmad Shah, 53, and co-pilot Fariq Abdul Hamid, 27. Both have been described as respectable, community-minded men.
Mr Zaharie joined Malaysia Airlines in 1981 and had more than 18,000 hours of experience. His Facebook page showed an aviation enthusiast who flew remote-controlled aircraft, posting pictures of his collection, which included a lightweight twin-engine helicopter and an amphibious aircraft.
Mr Fariq was contemplating marriage after having just graduated to the cockpit of a Boeing 777. He has drawn scrutiny after the revelation that in 2011, he and another pilot invited two women aboard their aircraft to sit in the cockpit for a flight from Phuket, Thailand, to Kuala Lumpur.
Fourteen countries are involved in the search, which is using 43 ships and 58 aircraft.
A US P-8A Poseidon, the most advanced long-range anti-submarine and anti-surface warfare aircraft in the world, was to arrive over the weekend and sweep parts of the Indian Ocean. It has a nine-member crew and has advanced surveillance and reconnaissance capabilities.