Investigators have expanded the search for a Malaysian airliner, missing for more than a week, deeper into the northern and southern hemispheres.

The Malaysian Defence Minister Hishammuddin Hussein announced on Monday that searches in both directions had begun, as Australia took the lead in scouring the southern Indian Ocean, and Kazakhstan responded to a Malaysian request for assistance in the unprecedented hunt.

"The fact that there was no distress signal, no ransom notes, no parties claiming responsibility, there is always hope," Hishammuddin said.

The co-pilot of the jetliner spoke the last words heard from the cockpit, according to the airline's chief executive. Investigators are reportedly considering suicide by the captain or first officer as one possible explanation for the disappearance.

Over the weekend, Malaysian Prime Minister Najib Razak announced that a satellite had picked up a faint signal from the aircraft about seven and a half hours after takeoff.

The signal indicated that the plane would have been somewhere on a vast arc stretching from Kazakhstan down to the southern reaches of the Indian Ocean.

Malaysia has asked for help from all countries in the search area and has requested access to radar data to narrow the task, the AP news agency reported.

Twenty-six countries are already involved in the search, which was initially concentrated on both sides of peninsular Malaysia.

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Had the plane gone northwest to Central Asia, it would have crossed over countries with busy airspaces, and some experts believe the person in control of the aircraft would more likely have chosen to go south. However, authorities are not ruling out the northern corridor.

The northern search corridor crosses through countries including China, India and Pakistan all of which have indicated they have seen no sign of the plane so far.

An official with the Chinese civil aviation authority said the missing plane did not enter Chinese airspace.

'Scouring data' 

To the south, Indonesian officials have said the plane did not cross their territory, based on radar data, AP reported.

An Air force spokesman said his country's search efforts were focusing on waters west of Sumatra in the Indian Ocean.

Australia has sent two AP-3C Orion aircraft, one of which is searching north and west of the Cocos Islands in the southern Indian Ocean, and plans to deploy two more by midday Tuesday.

Australia's defense department is refusing to say whether Malaysia has asked Australia to divulge any radar
information, but Prime Minister Tony Abbott said all Australian agencies are "are scouring their data to see if there's anything they can add to the understanding of this mystery".

Investigators have not ruled out hijacking or sabotage, and are checking the backgrounds of all 227 passengers and 12 crew members, as well as the ground crew, to see if links to armed groups, personal problems or psychological issues could be factors.

Source: Agencies