SYDNEY/KUALA LUMPUR (Reuters) - Australian officials leading the search for a missing Malaysia Airlines plane in the southern Indian Ocean are weighing when to deploy an underwater robot to aid in the hunt, now in its sixth fruitless week.

Malaysia Airlines Flight MH370 disappeared soon after taking off on March 8 from Kuala Lumpur bound for Beijing with 227 passengers and 12 crew on board, triggering a multinational search that is now focused on the Indian Ocean.

Searchers are confident they know the approximate position of wreckage of the Boeing 777, some 1,670 km (1,040 miles) northwest of Perth, after picking up several acoustic signals they believe are from its black box recorders.

With the batteries on the locators now two weeks past their 30-day expected life, the focus may soon shift to trying to find wreckage using sonar and cameras on a small unmanned "robot" known as an Autonomous Underwater Vehicle.

Two ships, one towing a sophisticated "towed pinger locator," are still criss-crossing the zone where four signals or pings were picked up, but the last was almost a week ago.

"This work continues in an effort to narrow the underwater search area for when the Autonomous Underwater Vehicle is deployed," the Australian agency heading the search said in a statement on Sunday.

The mystery has sparked what is on track to be the most difficult and expensive search and recovery operation in aviation history.

"Trying to locate anything 4.5 kilometers beneath the surface of the ocean about 1,000 kilometers from land is a massive, massive task and it is likely to continue for a long time to come," Australian Prime Minister Tony Abbott said at the weekend.

CO-PILOT CALL?

An aircraft's black box records data from the cockpit and conversations among flight crew and may provide answers about what happened to the missing plane, which flew thousands of kilometers (miles) off course after taking off.

Investigators probing the disappearance suspect that the co-pilot tried to make a call with his cellphone after the plane was diverted from its scheduled route, Malaysia's New Straits Times reported sources as saying on Saturday.

Government officials could not immediately be reached for comment on the report. The New Straits Times quoted acting Transport Minister Hishammuddin Hussein as saying that the report needed to be verified.

But he appeared to cast doubt on the report by saying: "If this did happen, we would have known about it earlier".

Malaysia is focusing its criminal investigation on the cabin crew and the pilots of the plane -- 53-year-old captain Zaharie Ahmad Shah and 27-year old Fariq Abdul Hamid -- after clearing all 227 passengers of any involvement, police have said.

Malaysia's government has also begun investigating civil aviation and military authorities to determine why opportunities to identify and track the flight were missed in the chaotic hours after it vanished.

Tony Abbott