PERTH (AFP) - A mini-sub searching for missing flight MH370 was again sweeping the Indian Ocean seabed on Wednesday after aborting its first mission, officials said, as Malaysia vowed to reveal any black box data found.

The unmanned submarine equipped with sonar gear was deployed on Tuesday night after data analysis of the first failed foray drew a blank, Australia's Joint Agency Coordination Centre (JACC) said.

After more than three weeks of hunting for black box signals, the autonomous sub had been deployed for the first time on Monday night from the Australian ship Ocean Shield, which has spearheaded the hunt for the Boeing 777 that vanished on March 8 with 239 people aboard.

"The autonomous underwater vehicle was again deployed last night (Tuesday) from ADV (Australian Defence Vessel) Ocean Shield," JACC said.

"The data from Bluefin-21's first mission has been downloaded and analysed. No objects of interest were found," JACC said.

The United States Navy explained that the Bluefin-21 had automatically aborted its first mission after six hours upon breaching its maximum operating depth.

JACC added that it had "exceeded its operating depth limit of 4,500m and its built-in safety feature returned it to the surface".

The sub was undamaged and had to be reprogrammed, said US Navy Captain Mark Matthews.

"In this case the vehicle's programmed to fly 30 metres over the floor of the ocean to get a good mapping of what's beneath," he told CNN from Perth after the aborted dive.

"It went to 4,500m and once it hit that max depth, it said 'This is deeper than I'm programmed to be', so it aborted the mission."

"To account for inconsistencies with the sea floor, the search profile is being adjusted to extend the sonar search for as long as possible," the US Navy statement said.

JACC chief Angus Houston announced on Monday the end of listening for signals from the plane's black boxes and launch of the submarine operation.

The mini-sub is supposed to conduct a sonar survey of the silty ocean floor for 16 hours at a time looking for wreckage from the Malaysia Airlines flight which disappeared en route from Kuala Lumpur to Beijing.

The US Navy estimated it would take the Bluefin-21 from six weeks to two months to scan the search zone, which has been deduced using satellite data and the detection of electronic pulses linked to black box recorders which were last heard a week ago.

Mr Houston has described the detections as the best lead in the hunt for the plane, and added Monday that an oil slick had also been sighted in the search area.

It would take several days to test a sample of the oil ashore, but Houston said he did not think it was from one of the many ships involved in the hunt.

The cause of the plane's 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.

Mr Houston has stressed the enormous difficulties of working at great depths in such a remote location and cautioned about the difficulties of finding the black boxes.

If they are ever found, Malaysia's transport minister pledged on Tuesday to make public any data recovered, as the government battles widespread criticism over the transparency of its investigation.

"It's about finding the truth. And when we... find out the truth, definitely we have to reveal what's in the black box," Datuk Seri Hishammuddin Hussein told reporters.

"So there is no question of it not being released."

The Malaysian government has been tight-lipped about its ongoing investigation into the disappearance of the jet, adding to the anger and frustration of relatives.

It has come under fire for a seemingly chaotic initial response, while the scarcity of official information on MH370 has prompted questions over its transparency.

Mr Hishammuddin said at the weekend that Malaysia's attorney-general had been sent abroad to confer with the International Civil Aviation Organization and determine which country would have custody of the black box, if it is ever found.

But he shrugged off the importance of the custody issue on Tuesday.

"I don't think it's important who gets custody as far as I'm concerned," he told reporters.

The Malaysian authorities insist they are hiding nothing but need to be cautious on commenting on ongoing investigations.

Mr Hishammuddin also said an "international investigation team" that Malaysia plans to set up to probe MH370's disappearance would be transparent and operate in accordance with international standards.