Australian PM confident signals are from missing plane

This chart released by Australia's Joint Agency Coordination Center shows search area being undertaken April 11, 2014.

Batteries fading as search closes in

Search efforts are now focused on three areas.

Aircraft and ships are combing two large search zones, some 2,390 km (1,485 miles) northwest of Perth, for possible floating debris related to the crash.

But it is the much smaller search zone, just 600 sq km (232 sq miles, located about 1,670 km (1038 miles) northwest of Perth that has generated fresh optimism.

The smaller zone is near where the Ocean Shield picked up the acoustic signals and where dozens of sonobuoys capable of transmitting data to search aircraft via radio signals were dropped on Wednesday.

The batteries in the black boxes have already reached the end of their 30-day expected life, making efforts to swiftly locate them on the murky ocean floor all the more critical, Abbott said.

"We are now getting to the stage where the signal from what we are very confident is the black box is starting to fade and we are hoping to get as much information as we can before the signal finally expires," he said.

But experts say the process of teasing out the signals from the cacophony of background noise in the sea is a slow and exhausting process.

An autonomous underwater vehicle named Bluefin-21 is onboard the Ocean Shield and could be deployed to look for wreckage on the sea floor once a final search area has been identified.