A robotic submarine deployed to search for the missing Malaysia Airlines plane in the southern Indian Ocean has had its first mission cut short.
The Bluefin-21 was sent to search the sea floor for wreckage after signals believed to be consistent with "black box" flight recorders were detected.
But the drone exceeded its operating limit of 4,500m (15,000ft) and was brought back to the surface.
It was due to return later on Tuesday if weather conditions permitted.
"To account for inconsistencies with the sea floor, the search profile is being adjusted to extend the sonar search for as long as possible," an update from the US Navy - which operates the Bluefin-21 - said.
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 thousands of kilometres off course, in seas west of the Australian city of Perth.
'Operating depth'Amid a major international search, an Australian navy vessel last week detected four acoustic signals using a US Navy towed pinger locator. Officials believe these could come from the missing plane's flight recorders.
No signals have been detected since 8 April, however, leading to fears that the recorders' batteries - which last about a month - have run out.
On Monday, Air Chief Marshal Angus Houston, who heads the agency co-ordinating the search, said it was time to deploy the Bluefin-21 to begin its search of the sea floor. It set off on its first mission on Monday night.
"After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500m and its built in safety feature returned it to the surface," the Joint Agency Co-ordination Centre said in a statement.