The hunt by a mini-submarine for a Malaysian airliner in the uncharted depths of the Indian Ocean -- the latest phase in a huge international search -- will end in about a week, Australia says.
If the best leads now being pursued to locate wreckage from the Boeing 777 prove fruitless, said Prime Minister Tony Abbott, the search would not be abandoned but enter a new phase.
The US Navy's Autonomous Underwater Vehicle (AUV) Bluefin-21 is the newest tool deployed in the hunt, after satellite data and electronic signals from the plane's black box helped narrow the search area.
But the unmanned torpedo-shaped sonar mapping device is operating at the extreme limit of its range, and has twice had to cut short its mission.
"We believe that (sub) search will be completed within a week or so," Abbott told Thursday's Wall Street Journal in an interview as the device mapped the seabed 2,000 kilometres (1,240 miles) off the western Australian city of Perth.
"If we don't find wreckage, we stop, we regroup, we reconsider."
The AUV Thursday completed its first full mission at the third attempt, officials said, and was readying to dive again. Data it retrieved was being analysed.
The first two part-completed missions failed to produce any results.
Malaysia Airlines Flight MH370 vanished on March 8 on a flight from Kuala Lumpur to Beijing with 239 people on board.
The cause of its disappearance, after being diverted hundreds of kilometres off course, remains a mystery. No debris has been found despite an enormous search involving ships and planes from several nations.
"My determination for Australia is that we will do whatever we reasonably can to resolve the mystery," Abbott said.
"If the current search turns up nothing, we won't abandon it, we will simply move to a different phase," he was quoted by the Journal as saying, reiterating his confidence that searchers were looking in the right place.
After more than three weeks of listening for signals from the black box flight recorder, the sub was deployed for the first time on Monday night but aborted the mission after breaching its 4,500 metre (15,000 feet) operating limit.
An unexplained technical problem cut short the second mission, raising questions about the whole search, which is expected to be further complicated by thick silt on the seabed.
The Bluefin is being deployed from the deck of the Australian vessel Ocean Shield, which has led the search.
Before Abbott's comments the US Navy estimated it would take the Bluefin-21 "anywhere from six weeks to two months to scan the entire search area".
Australia's Joint Agency Coordination Centre (JACC) has repeatedly called for patience and warned the search will be a long, laborious process.
"Bluefin-21 has searched approximately 90 square kilometres (35 square miles) to date and the data from its latest mission is being analysed," JACC said.
JACC chief Angus Houston has stressed that the mini-sub cannot operate below 4,500 metres and that other vehicles would have to be brought in to cope with greater depths.
Experts have said these could include remotely operated vehicles (ROVs) that can go as deep as six kilometres.
An ROV was used to pluck the flight data recorders of Air France 447 from the bottom of the Atlantic in 2011.
Houston had announced Monday the end of listening for electronic pulses signals from the black box and the launch of the submarine operation.
The mini-sub is supposed to conduct a sonar survey of the ocean floor for 16 hours at a time. The dive itself takes two hours as does re-surfacing.
Houston has described the detection of the electronic pulses, last heard more than a week ago, as the best lead in the hunt for the plane.
An oil slick had also been sighted in the search area, he said Monday, and was taken ashore by sea for testing to see whether it came from the plane.
JACC said Thursday the oil sample had arrived in Perth for analysis.
Experts stress that the search area is a dark, extremely deep and little-known seascape and a daunting prospect.
"It has not been mapped -- in fact most of the deep ocean has not been mapped," said Charitha Pattiaratchi, an oceanographer at the University of Western Australia.
"It is very cold and dark with high pressures -- 450 times that at the surface."
The visual search for debris also continued Thursday, JACC said, with as many as 12 aircraft and 11 ships involved over an area of 40,349 square kilometres (15,579 square miles) more than 2,170 kilometres (1,345 miles) northwest of Perth.