The Australian authority leading the search for missing Malaysia Airlines Flight MH370 said Friday that "hard spots" had been found on the Indian Ocean seabed, but that most would likely be geological features.

Experts are conducting a sonar survey of a remote patch of the southern Indian Ocean, an area never previously explored in such detail, in preparation for an underwater search for the plane which disappeared on March 8 with 239 people onboard.

The Australian Transport Safety Bureau said the sonar search had provided information on the depth of the water and the composition of the sea floor in the search zone.

"The multibeam sonar can identify degrees of hardness, although it cannot distinguish between (for example) the hard metal of an aircraft and the hard rock of the seafloor," an ATSB spokesman said.

"The vast majority of hard spots found are most likely to be geological features as opposed to man-made objects."

Flight MH370 went missing en route from Kuala Lumpur to Beijing and the seabed mapping has already uncovered previously unknown volcanoes on the ocean floor.

The ATSB said by identifying the hard objects, experts were "informing where closer investigation may be required during the deep water search".

The plane is believed to have crashed into the southern Indian Ocean far off the west coast of Australia after mysteriously diverting off-course, but a massive air, sea and underwater search has failed to find any wreckage.

Experts have used technical data to finalise its most likely resting place deep under the Indian Ocean and are preparing for a more intense underwater search.

Likely to start this month, this will focus on a dauntingly vast stretch of ocean measuring 60,000 square kilometres (23,000 square miles).

Last month Australian Deputy Prime Minister Warren Truss said the ongoing mapping of the ocean floor had already uncovered "quite remarkable" geographical features, including the discovery of new volcanoes up to 2,000 metres (6,562 feet) high.

"In one place in particular... the sea depth is as little as 600 metres, and then falls away in just a very short distance to 6,600 metres," he said.

"It's quite dramatic in some places, and certainly the equipment could be lost if we had not earlier been able to map it and detect where these mountains are, where the trenches are, and where areas can be searched with the confidence that the equipment will not be damaged."