"DELIBERATE" was the one word that stood out in the most recent statements made by authorities about the fate of the missing Malaysia Airlines jetliner.

Malaysian Prime Minister Najib Razak said Flight MH370's communication systems were "deliberately" shut down and the plane was "deliberately" flown off course by someone on board who had knowledge of aviation.

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It was by far the most definitive statement made since investigators began looking into reasons why the Beijing-bound plane vanished from radar screens exactly a week ago today and had remained hidden from search and rescue teams that had been scouring the seas and jungles to find it.

The Malaysian leader's statement corroborates what US investigators earlier described as a possible "human intervention" in shutting down the plane's two communication systems separately, several minutes apart.

The US experts said it could not have been a "catastrophic accident."

The Prime Minister categorically said the missing MH370 was not an accident and that they are investigating all angles including the backgrounds of the flight passengers and crew members.

Already, security has been tightened around the house of the pilot, Razak said.

Suspicions of hijacking ran high as the Prime Minister said that evidence point directly to the fact that someone else shut down the airline's internal and external communication systems "deliberately."

Evidence is "consistent with the deliberate action of someone on the plane," Razak said, adding that the investigation is "entering a new phase".

According to Razak, there is a "high degree of certainty" that the plane's Communications Addressing and Reporting System was disabled just before it reached the east coast of Malaysia.

Razak pointed out that  shortly afterwards, near the border between Malaysian and Vietnamese air traffic control, the aircraft's transponder was switched off.

The plane kept pinging a satellite up to 8:11am local time, Razak said, and the satellite communication data confirmed that the plane turned from its original course.

"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," said Razak.

"Aviation authorities now believe the plane's last communication with the satellite was either in a northern corridor between the border of Kazakhstan and Turkmenistan to northern Thailand, or a southern corridor from Indonesia to the southern Indian Ocean,"  Razak added.

Chinese scientists, meantime, said they recorded "seismic activities" on the seabed between the Malaysia and Vietnam waters at the time the jetliner disappeared from radar screen, indicating the missing Malaysia Airlines flight MH370 may have crashed into the sea.

A research team of the University of Science and Technology of China said the simultaneous occurrence of the seismic activities and the plane's disappearance intensified suspicions of a plane crash.

"The incident happened at 2.55am on 8 March, about one-and-a-half hours after MH370 lost contact at 1.30am.  The seafloor event could have been caused by the plane possibly plunging into the sea. The strength of the earthquake wave indicates the plunge was catastrophic," the research group said.

Search and rescue operations have expanded from the South China Sea to the Bay of Bengal and Indian Ocean.