The plane continued flying for around seven hours after losing contact with the ground and could have gone towards Pakistan.
Speaking for the first time about the incident, Malaysia's Prime Minister Najib Razak yesterday said investigators believe somebody cut off the plane's communications and steered it west, far from its scheduled route to Beijing.
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.
Prime Minister Razak stopped short of confirming this during a televised news conference, saying: "We are still investigating all possibilities as to what caused MH370 to deviate."
Minutes after his statement ended, police officers arrived at the home of Zaharie Ahmad Shah, the 53-year-old captain of the plane, on the outskirts of Kuala Lumpur.
The Malaysia Airlines MH370, a Boeing 777-200ER, vanished from radar screens more than a week ago with 239 crew and passengers aboard. It took off from Kuala Lumpur last Friday and was supposed to land in Beijing just under six hours later.
Razak's statement confirmed days of mounting speculation that the disappearance of MH370 was not accidental. It refocused the investigation into the flight's crew and passengers and underlined the massive task for searchers who have already been scouring vast areas of ocean.
"Clearly the search for MH370 has entered a new phase," Razak said.
"In view of this latest development, the Malaysian authorities have refocused their investigation into the crew and passengers on board."
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. Apart from hijacking, one possibility they have raised was that one of the pilots wanted to commit suicide.
The plane departed for an overnight flight from Kuala Lumpur to Beijing at 12.40am local time 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.
The last verbal communication with the plane came at the boundary between Malaysian and Vietnamese airspace. Told by Malaysia's air traffic control that the flight was being passed to Ho Chi Minh control, the crew replied: "OK, roger that."
Investigators now have a high degree of certainty that one of the plane's communications systems - the Aircraft and Communications Addressing and Reporting System (ACARS) - was disabled before the aircraft reached the east coast of Malaysia, Razak said. It is a service which allows computers on the plane to relay in-flight information about the health of its systems to the ground.
Shortly afterwards, someone on board then switched off the aircraft's transponder, which communicates with civilian air traffic controllers.
Razak confirmed that Malaysian air force defence radar then picked up signals 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," he added.
Although the aircraft was flying virtually blind to air traffic controllers at this point, on board 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 - seven 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," Razak 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.
David Gleave, an aviation safety investigator, said one other credible scenario was that someone on board had crashed the aeroplane, effectively committing suicide.
But he said there was also the possibility the aircraft had been hijacked and landed somewhere.
Suggesting one possible scenario, Gleave said that whoever had taken over the plane would have had to "get all the communication devices off the passengers as well and then there is ransom demands going on that aren't in the public domain," he suggested. "So the aircraft is worth well over $50 million (£30 million) and then each of the passengers' lives is say $1m (£600,000). There could be negotiations with embassies and the insurance companies and that wouldn't necessarily be in the public domain."
But he added: "However, to hide the aeroplane you would need to have done a lot of planning beforehand for this."
The northern route described by Razak might have taken the plane through a region home to extremist Islamist groups and unstable governments, as well as remote, sparsely populated areas. However, 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 between 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.
Razak 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.
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.
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.
But at least one of the people waiting at the hotel saw a glimmer of hope with claims now 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 Shah and his co-pilot Fariq Abdul Hamid, 27.
Both have been described as respectable, community-minded men.
Shah 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. He spent his off days tinkering with a flight simulator of the plane that he had set up at home, current and former co-workers said.
Hamid was contemplating marriage after having just graduated to the cockpit of a Boeing 777. He has been portrayed as a cockpit Romeo 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. But family and friends counteracted these reports, saying he was religious and serious about his career.
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 due arrive this weekend and sweep parts of the Indian Ocean. It has a nine-member crew and has advanced surveillance and reconnaissance capabilities.
As the search enters its second week, several governments are using imaging satellites that take high definition photos - while data from private sector communications satellites is also being examined. China alone says it has deployed 10 satellites.
"The area is enormous. Finding anything rapidly is going to be very difficult," said Marc Pircher, director of the French space centre in Toulouse. "The area and scale of the task is such that 99% of what you are getting are false alarms."
The corridors given by Malaysian PM Razak represent a satellite track, which appears as an arc on a map. The plane did not necessarily follow the corridor, but was at some point along its path at the moment the signal was sent.
Earlier, a source familiar with official US assessments of electronic signals sent to geostationary satellites operated by Britain's Inmarsat said it appeared most likely the plane turned south over the Indian Ocean, where it would presumably have run out of fuel and crashed into the sea.
If so, just finding the plane - let alone recovering the "black box" data and cockpit voice recorders that hold the key to the mystery - would be a huge challenge.
The expanse of the Indian Ocean has an average depth of more than 12,000 feet, or two miles - deeper than the Atlantic, where it took two years to locate wreckage on the seabed from an Air France plane that vanished in 2009 even though floating debris quickly pointed to the crash site.
Any debris would have been widely dispersed by Indian Ocean currents in the week since the plane disappeared.
"We have many radar systems operating in the area, but nothing was picked up," said India's Rear Admiral Sudhir Pillai, Chief of Staff of Andamans and Nicobar Command.
"It is possible that the military radars were switched off as we operate on an as-required basis.
"So perhaps secondary radars were operating which may not have the required range to detect a flight at an altitude of 35,000 feet."
Another possible scenario is that the aircraft continued to fly to the northwest and headed over Indian territory.
But the source said it was believed unlikely the plane flew for any length of time over India because it has strong air defence and radar coverage and that should have allowed authorities there to see the plane and intercept it.