The investigation into what happened to Malaysia Airlines Flight 370 may never determine the cause of the tragedy, Malaysia's national police chief has warned.

Khalid Abu Bakar said today the criminal investigation was still focused on four areas - hijacking, sabotage and personal or psychological problems of those on board the plane.

"Investigations may go on and on and on. We have to clear every little thing," he said.

"At the end of the investigations, we may not even know the real cause. We may not even know the reason for this incident."

The plane disappeared on March 8 on a flight to Beijing from Kuala Lumpur with 239 on board. No trace of the plane has been found, although searchers are now focused on a crash site in the southern Indian Ocean.

Mr Khalid said that police had conducted more than 170 interviews with family members of the pilots and crew members.

"We must be very thorough and we need all the time ... you cannot hurry us," he said.

Police are also investigating the cargo and even the food served on the plane to eliminate possible sabotage, he said.

Relatives of the passengers have been critical of the handling of the search for the plane, especially as the focus of the hunt has shifted.

After experts analysed the limited radar and satellite data from the plane, the search area was moved from the seas off Vietnam, to several areas in the Indian Ocean west of Australia, and finally to an 85,000-square-mile area roughly a two-and-a-half-hour flight from Perth.

The search resumed today, with the first of nine planes heading out to the search zone, about 930 miles west of Perth, the Australian Maritime Safety Authority said. Nine ships were also scouring the area.

Australian prime minister Tony Abbott reiterated his commitment to the search ahead of meeting his Malaysian counterpart Najib Razak later today in Perth.

"Look, it's one of the great mysteries of our time," Mr Abbott told Perth Radio 6iX.

"It's a terrible tragedy. There are 239 devastated families. ... We owe it to the world, we owe it to those families to do whatever we reasonably can do get to the bottom of this."

Mr Najib's trip to Perth also will include a visit to the joint agency co-ordinating the multinational search effort.

Angus Houston, who heads the agency, said no time frame has been set for the search for wreckage to end.

"Over time, if we don't find anything on the surface, we're going to have to think about what we do next, because clearly it's vitally important for the families, it's vitally important for the governments involved that we find this airplane," he said.

Mr Houston said it was only once wreckage from the plane was found that "we will then be able to narrowly focus the search area so that we can start to exploit the underwater technology devices that will hopefully lead to where the aircraft is on the bottom of the ocean".