Media playback is unsupported on your device Media caption ACM Angus Houston: Bluefin-21 will create a 3D map of the ocean floor 
 
 Teams searching for the missing Malaysia Airlines plane are to deploy a robotic submarine for the first time. 
 
 Search chief Angus Houston said the Bluefin-21 drone would be sent down as soon as possible to search for wreckage on the sea floor. 
 
 Teams have been using a towed pinger locator to listen for signals from the plane's "black box" flight recorders. 
 
 But no new signals have been heard since 8 April, amid concerns the flight recorders' batteries have expired. 
 
 Flight MH370 went missing on 8 March with 239 people on board. It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers over the South China Sea. 
 
 Malaysian officials believe, based on satellite data, that it ended its flight in the southern Indian Ocean, thousands of kilometres off course. 
 
 'Manageable search area' 
 
 An international search has focused on waters west of the Australian city of Perth, with teams racing against time to detect signals before the flight recorder batteries - which last about one month - run out. 
 
 Image copyright Getty Images Image caption The Bluefin-21 will search the sea floor for wreckage from flight MH370 
 
 Image copyright Reuters Image caption Up to 12 planes and 15 ships were involved in the search for the plane 
 
 Air Chief Marshal Houston, who heads the joint agency co-ordinating the search effort, said that given no signals had been detected in six days, it was time to go underwater. 
 
 The Bluefin-21 - an almost 5m-long underwater autonomous vehicle that can create a sonar map of the sea floor - will search for wreckage in an area defined by four signals heard last week. 
 
 Officials believe those signals - picked up by the pinger locator towed by an Australian vessel - are consistent with flight recorders. 
 
 "Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area on the ocean floor," ACM Houston said. 
 
 "The experts have therefore determined that the Australian Defence Vessel Ocean Shield will cease searching with the towed pinger locator later today and deploy the... Bluefin-21 as soon as possible.'' 
 
 He warned that the submersible search would be a long, "painstaking" process that might, in the end, yield no results. 
 
 Each Bluefin-21 mission will last 24 hours, with 16 hours spent on the ocean floor, four hours' diving and resurfacing time, and four hours to download data. 
 
 MH370 - Facts at a glance 8 March: Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears 
 
 Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears Plane's transponder, which communicates with ground radar, was switched off as it left Malaysian airspace 
 
 Satellite 'pings' indicate plane was still flying seven hours after satellite contact was lost 
 
 24 March: Based on new calculations, Malaysian PM says "beyond reasonable doubt" that plane crashed in southern Indian Ocean with no survivors What we know The search for flight MH370 
 
 The Ocean Shield had also spotted an oil slick in the same area where the signals had been heard, ACM Houston said, and a sample was being sent for testing. 
 
 "I stress the source of the oil is yet to be determined but the oil slick is approximately 5,500m downwind... from the vicinity of the detections picked up by the towed pinger locator," he said. 
 
 Australian officials have said previously that they are confident they are searching in the right area for the missing plane. 
 
 Officials have no idea yet why the plane diverted so far from its intended flight path. Investigators are looking at options including hijacking, mechanical failure, sabotage and pilot action. 
 
 But recovering the flight recorders is seen as key to understanding what happened to the plane. ||||| 1 of 7. A fast response craft from Australian Defence Vessel Ocean Shield tows Able Seaman Clearance Diver Michael Arnold as he searches the ocean for debris in the search zone in the southern Indian Ocean for the missing Malaysian Airlines flight MH370 in this picture released by the Australian Defence Force April 8, 2014. 
 
 SYDNEY/PERTH, Australia (Reuters) - An Australian ship which picked up possible "pings" from the black box recorders of a missing Malaysia Airlines jetliner has been unable to detect any further signals and time is running out to narrow the massive search, officials said on Tuesday. 
 
 Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight MH370, said the month-long hunt in the Indian Ocean was at a critical stage given the batteries in the black box beacons had already reached the end of their 30-day expected life. 
 
 A U.S. Navy "towed pinger locator" onboard Australia's Ocean Shield picked up two signals consistent with black box locator beacons over the weekend - the first for more than two hours and the second for about 13 minutes. 
 
 Houston said the signals represented the best lead in the search yet, but efforts to pick up the pings again had so far been unsuccessful. 
 
 "If we don't get any further transmissions, we have a reasonably large search area of the bottom of the ocean to prosecute and that will take a long, long time. It's very slow, painstaking work," said Houston. 
 
 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. 
 
 Authorities say evidence, including the loss of communications, 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 an area some 1,680 km (1,040 miles) northwest of Perth, near where possible pings were picked up and the search is now focused. 
 
 BLUEFIN ON HOLD 
 
 An autonomous underwater vehicle (AUV) named Bluefin-21 is onboard the Ocean Shield and could be sent to look for wreckage on the sea floor, but narrowing the search zone first was critical, Houston said. 
 
 "It is a large area for a small submersible that has a very narrow field of search, and of course, it is literally crawling along the bottom of the ocean," he said. 
 
 "That's why it's so important to get another transmission and we need to continue until there's absolutely no chance the (black box) device is still transmitting." 
 
 The Bluefin will scour the ocean floor in 20-hour missions using sonar in an attempt to find the aircraft, before its findings are downloaded and analyzed on board the Ocean Shield. 
 
 If anything unusual is spotted, the sonar on board the robotic vehicle will be replaced with a camera to take a closer look. The potential search area is about 4.5 km (2.8 miles) deep, the outer reach of the Bluefin's range. 
 
 Some 133 missions have been completed so far in the multinational aerial hunt for debris in the southern Indian Ocean and would continue, officials said. 
 
 Up to 11 military planes, three civilian planes and 14 ships planned to take part in the search on Tuesday, with good weather in the search area. But so far, searchers have only turned up fishing gear and other detritus. 
 
 "Even after the black box is found, the surface search will continue because they're looking for evidence to investigate why the aircraft went down," Lieutenant Commander Adam Schantz, the officer in charge of U.S. Navy P-8 Poseidon search unit, told Reuters. 
 
 "So, we'll continue for a while trying to find anything on the surface that might give a reasoning behind the accident." 
 
 (Additional reporting by Jane Wardell in Sydney and Anuradha Raghu in Kuala Lumpur; Editing by Dean Yates and Michael Perry)
– Australian officials have sounded a fresh note of confidence in the search for Malaysia Airlines Flight 370, saying they've reacquired underwater signals following a lapse into silence. Search chief Angus Houston says an Australian navy vessel detected two sets of pings yesterday in the same area that signals were detected over the weekend, reports the Sydney Morning Herald. "I'm now optimistic that we will find the aircraft, or what is left of the aircraft, in the not too distant future—but we haven't found it yet, because this is a very challenging business," Houston told reporters. The signals are getting weaker, which could be a sign that searchers are getting further away, or, more likely, a sign that the data recorder batteries are starting to fade, explained Houston. He says they have delayed deploying the autonomous underwater vehicle Bluefin-21 to search for wreckage because with time running out, they don't want anything to get in the way of the signals. In another promising development, authorities have analyzed the pings detected over the weekend and confirmed that they came from electronic equipment, with signals consistent with those of flight data recorders, CNN reports.