The hunt for missing Malaysia Airlines flight MH370 moved into a new phase on Friday with the deployment of a towed pinger locator to find the black box.
Two ships with locator capabilities are searching a 240km (150 mile) path in a bid to retrieve the data recorder.
But Australia's search chief said it was a race against time as the battery-powered signal fades after 30 days.
The plane disappeared on 8 March en route from Kuala Lumpur to Beijing. It was carrying 239 people.
It is believed to have crashed in the southern Indian Ocean, although no confirmed debris has been found from the plane.
The search is being co-ordinated from the city of Perth in Western Australia.
Fourteen aircraft and 11 ships were involved in Friday's search activities.
Ships sighted a number of objects in the area but none were associated with the missing plane, the coordination agency said.
Angus Houston, head of the Joint Agencies Coordination Centre (JACC) leading the search, said that two ships had "commenced the sub-surface search for emissions from [the] black box pinger".
Australia naval vessel Ocean Shield was using a towed pinger locator from the US Navy, while HMS Echo, which had similar capabilities, was also searching.
But the former military chief now coordinating the eight-nation search said it was "getting pretty close to the time when it might expire."
Beacons in the black box emit "pings" so they can be more easily found but the batteries only last for about a month.
"The two ships will search a single 240km track converging on each other," Air Chief Marshal Houston, who is retired, said.
ACM Houston said that the area had been picked on the basis of analysis of the satellite data.
It was based on work regarding "how the aircraft might have performed and how it might have been flown", to choose the "area of highest probability as to where it might have entered the water".
He pointed out that this data was continuing to be refined, but the current search was based on the "best data that is available".
Given the progress in data evaluation and calculation, "there is some hope we will find the aircraft in the area we are searching", he added.
The two ships will be moving at reduced speeds, of around three knots, in their attempt to detect any signal from the pinger.
Commodore Peter Leavy, Commander of Joint Task Force 658, said that search operations generally preferred to use "physical evidence" and "drift modelling" to locate a plane.
However, "no hard evidence has been found to date so we have made the decision to search a sub-surface area on which the analysis has predicted MH370 is likely to have flown," he said.
In a statement, JACC said the focus was now on a search area of about 217,000 sq km (84,000 sq miles), 1,700 km (1,000 miles) north west of Perth.
Meeting staff involved in the search on Friday, Australian Prime Minister Tony Abbott said: "It is probably the most difficult search that's ever been mounted."
"A large aircraft seems like something that would be easy enough to locate - but a large aircraft that all but disappeared and disappeared into inaccessible oceans is an extraordinary, extraordinary challenge that you're faced with."
ACM Houston said there was still a "great possibility of finding something on the surface [of the ocean]".
"There's lots of things in aircraft that float," he said, citing previous searches where life jackets from planes were found.