Photo: CTV

In this Monday, April 14, 2014, photo provided by the Australian Defense Force an autonomous underwater vehicle is deployed from ADV Ocean Shield in the search of the missing Malaysia Airlines Flight 370 in the southern Indian Ocean. (AP Photo/Australian Defense Force, Lt. Kelli Lunt)

A robotic submarine hunting for the missing Malaysian jet aborted its first mission after only six hours, surfacing with no new clues when it exceeded its maximum depth along the floor of the Indian Ocean, officials said Tuesday.

Search crews sent the U.S. Navy's Bluefin 21 into the depths Monday to begin scouring the seabed for the missing Malaysia Airlines Boeing 777 after failing for six days to detect any new signals believed to be coming from its black boxes.

But the 16-hour mission was cut short when the unmanned sub, which is programmed to hover 30 metres (100 feet) above the seabed, entered a patch that was deeper than its maximum depth of 4,500 metres (15,000 feet), the search co-ordinationcentre and the U.S. Navy said.

A built-in safety feature returned the Bluefin to the surface and it was not damaged, they said.

The data collected by the sub was later analyzed and no sign of the missing plane was found, the U.S. Navy said. Crews were shifting the Bluefin's search area away from the deepest water and were hoping to send it back on another mission later Tuesday.

Search authorities had known the primary search area for Flight 370 was near the limit of the Bluefin's dive capabilities. Deeper-diving submersibles have been evaluated, but none is yet available to help.

A safety margin would have been included in the Bluefin's program to protect the device from harm if it went a bit deeper than its 4,500-meter limit, said Stefan Williams, a professor of marine robotics at the University of Sydney.

"Maybe some areas where they are doing the survey are a little bit deeper than they are expecting," he said. "They may not have very reliable prior data for the area."

Meanwhile, officials were investigating an oil slick about 5,500 metres (3.4 miles) from the area where the last underwater sounds were detected.

Crews collected an oil sample and sent it back to Perth in western Australia for analysis, a process that will take several days, said Angus Houston, the head of the joint agency co-ordinating the search off Australia's west coast.

He said it does not appear to be from any of the ships in the area, but cautioned against jumping to conclusions about its source.

On Tuesday, Malaysia's defence minister, Hishamuddin Hussein, pledged to reveal the full contents of the black boxes if they are found.

"It's about finding out the truth," he told reporters in Kuala Lumpur. "There is no question of it not being released."

Up to 11 planes and as many ships were scouring a 62,000-square kilometre (24,000-square mile) patch of ocean about 2,200 kilometres (1,400 miles) northwest of Perth on Tuesday, hunting for any floating debris.

The weekslong surface search is expected to end in the next two days. Officials haven't found a single piece of debris confirmed to be from the plane, and Houston said the chances that any would be found have "greatly diminished."