(Adds officials warn of long search, quotes and details, paragraphs 4-8, 12-16)
* Final cockpit words "Good night Malaysian three seven zero"
* Changed account of final words comes in fourth week of search
* Ten planes, nine ships resume hunt west of Perth
* Australian officials stress challenges of search
* Malaysia PM heading to Perth to visit search base
By Stuart Grudgings and Matt Siegel
KUALA LUMPUR/PERTH, April 1 (Reuters) - The last words from the cockpit of a missing Malaysian jet were a standard "Good night Malaysian three seven zero", Malaysian authorities said, changing their account of the critical last communication from a more casual "All right, good night."
The correction more than three weeks after Flight MH370 vanished with 239 people on board was made as Malaysian authorities face heavy criticism, particularly from China, for mismanaging the search and holding back information.
Painstaking analysis of radar data and limited satellite information has focused the search on a vast, inhospitable swath of the southern Indian Ocean west of the Australian city of Perth, but has so far failed to spot any sign of the jetliner.
Search coordinators warned the hunt could drag on for some time yet.
"In this case, the last known position was a long, long way from where the aircraft appears to have gone," retired Air Chief Marshal Angus Houston, the head of the Australian agency coordinating the operation, told reporters in Perth.
"It's very complex, it's very demanding and we don't have hard information like we might normally have," he said.
The Boeing 777 disappeared from civilian radar in the early hours of March 8 as it flew from Kuala Lumpur to Beijing. Minutes later its communications were cut off and it turned back across Malaysia and headed towards the Indian Ocean.
Malaysia says the plane was likely diverted deliberately, probably by a skilled aviator, leading to speculation of involvement by one or more of the pilots. Investigators, however, have determined no apparent motive or other red flags among the 227 passengers and 12 crew.
"We would like to confirm that the last conversation in the transcript between the air traffic controller and the cockpit is at 0119 (Malaysian Time) and is "Good night Malaysian three seven zero," the Department of Civil Aviation said in a statement late on Monday.
Malaysia's ambassador to China told Chinese families in Beijing as early as March 12 that the last words had been "All right, good night". About two-thirds of the passengers on board were Chinese.
The statement said authorities were still conducting "forensic investigation" to determine whether the last words from the cockpit were by the pilot or the co-pilot. Malaysia Airlines had previously said the words were believed to have come from the co-pilot.
SEARCH GOES ON
Nine ships and 10 aircraft resumed the hunt for wreckage from MH370 on Tuesday, hoping to recover more than fishing gear and other flotsam found since Australian authorities moved the search 1,100 km (685 miles) north after new analysis of radar and satellite data.
Houston said the challenging search, in an area the size of Ireland, would continue based on the imperfect information with which they had to work.
"But, inevitably, if we don't find any wreckage on the surface, we are eventually going to have to, probably in consultation with everybody who has a stake in this, review what to do next," he said.
Using faint, hourly satellite signals gathered by British firm Inmarsat PLC and radar data from early in its flight, investigators have only estimates of the speed the aircraft was travelling and no certainty of its altitude, Houston said.
Satellite imagery of the new search area had not given "anything better than low confidence of finding anything", said Mick Kinley, another search official in Perth.
Malaysian Prime Minister Najib Razak will travel to Perth on Wednesday to see the operations first hand.
Among the vessels due to join the search in the coming days is an Australian defence force ship, the Ocean Shield, that has been fitted with a sophisticated U.S. black box locator and an underwater drone.
Time is running out because the signal transmitted by the missing aircraft's black box will die about 30 days after a crash due to limited battery life, leaving investigators with a vastly more difficult task.
(Additional reporting by Michael Martina in PERTH and Jane Wardell in SYDNEY; Writing by Lincoln Feast; Editing by Paul Tait)