Two ships with sophisticated equipment for searching underwater have zeroed in on a remote stretch of the Indian Ocean in a desperate hunt for the missing Malaysia Airlines jet's black boxes, whose batteries will soon run out.

The ships with equipment that can hear the black boxes' pings were slowly making their way on Friday along a 240-km route that investigators are hoping may be close to the spot Flight 370 entered the water after it vanished on March 8 en route from Kuala Lumpur to Beijing.

The black boxes could provide crucial information about what condition the plane was flying under and any communications or sounds in the cockpit.

The head of the joint agency coordinating the search, however, acknowledged that the search area was essentially just a best guess, and noted that time was running out for search crews to find the coveted data recorders.

"The locater beacon will last about a month before it ceases its transmissions, so we're now getting pretty close to the time when it might expire," Angus Houston told the Associated Press news agency.

The Australian navy ship Ocean Shield, which is dragging a towed pinger locater from the US Navy, and the British navy's HMS Echo, which has underwater search gear on board, were looking for the black boxes in an area that investigators' settled on after analysing hourly satellite pings the aircraft gave off after it disappeared.

That information, combined with data on the estimated speed and performance of the aircraft, led them to that specific stretch of ocean, Houston said.

A weeks-long hunt has not turned up a single piece of wreckage, which could have led the searchers to the plane.

Pinger locater

Because the US Navy's pinger locater can pick up black box signals up to a depth of 6,100m, it should be able to hear the devices even if they are lying in the deepest part of the search zone, which is about 5,800m below the surface.

But that is only if the locator gets within range of the black boxes, a tough task, given the size of the search area and the fact the pinger locator must be dragged slowly through the water at just 1 to 5 knots, or 1.6 to 9.6 kilometres per hour.

Finding floating wreckage is key to narrowing the search area, as officials can then use data on ocean currents to try and backtrack to the spot where the Boeing 777 hit the water, and where the black boxes may be.

But with no wreckage found despite weeks of searching, officials cannot be confident that they are looking for the black boxes in the right place, said Geoff Dell, discipline leader of accident investigation at Central Queensland University in Australia.

"They might be lucky and they might start smack bang right over the top of it," Dell said. "But my guess is that on the balance of probabilities, that's not going to be the case and they're in for a lengthy search."

Flight MH370 had 239 people on board.

Source: Agencies