A U.S. Navy "towed pinger locator" (TPL) onboard Australia's Ocean Shield on the weekend picked up two signals consistent with black box locator beacons - the first for more than two hours and the second for about 13 minutes - at the weekend.

Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight MH370, said that the signals represented the best lead yet in the month-long hunt in the Indian Ocean.

But Commander William J. Marks, public affairs officer for the U.S. Navy's 7th Fleet, told Reuters on Wednesday that they had so far failed to replicate the findings, raising concerns as time ticks down on the devices' batteries.

"We have not been able to reacquire," Marks said.

The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometers off its Kuala Lumpur-to-Beijing route.

But the batteries in the beacons have already reached the end of their 30-day expected life, making efforts to swiftly locate them all the more critical.

Authorities say evidence suggests the plane was deliberately diverted by someone familiar with the aircraft, but have not ruled out mechanical problems.

Analysis of satellite data led investigators to conclude the Boeing 777 came down in a remote area of the Indian Ocean near Perth. The search is now centered on an area approximately 2,261 kilometers (1,405 miles) northwest of that city.

FRESH DOUBTS ABOUT VERACITY OF "PINGS"

Up to 11 military aircraft, four civilian aircraft and 14 ships are planned to take part in the search on Wednesday, with scattered showers expected in the search area.

An autonomous underwater vehicle named Bluefin-21 is onboard the Ocean Shield and could be sent to look for wreckage on the sea floor once intelligence narrows the search area.

The potential search area is currently about 4.5 km (2.8 miles) deep, the outer reach of the Bluefin's range.

A U.S. Navy officer on board the Ocean Shield outlined for Reuters a number of scenarios that could account for the inability to reacquire the "ping" signal.

Those include the possibility that the initial detection was false, that it was positive but the batteries have since run out or that it had caught a mere "whiff" of the signal and had since moved too far from the correct location to reacquire it.

The decision to hold back the underwater vehicle reflects both a hope that the batteries are still alive and the belief that the search area remains too large.

"I can cover in one day with TPL (towed pinger locator) the amount of area it will take me to cover with Bluefin in six days," the U.S. Navy officer said.

"So if I am deploying Bluefin now I am basically saying that I don't have confidence in the TPL that it will detect the signals anymore," he said.