(File picture) Crew on the Australian Defence Vessel Ocean Shield move the US Navy's Bluefin-21 autonomous underwater vehicle into position for deployment in the southern Indian Ocean to look for the missing Malaysia Airlines flight MH370 in 2014 in this handout picture released by the US Navy.

Sydney, Australia - Investigators will deploy an underwater vehicle to take a closer look at objects found during a sonar survey of the southern Indian Ocean in the hunt for MH370, Australia said on Wednesday, as it extended the search into next year.

Malaysia Airlines MH370, a Boeing 777, disappeared en route from Kuala Lumpur to Beijing on March 8, 2014 carrying 239 passengers and crew.

Despite an extensive underwater search far off Western Australia's coast where investigators believe the plane crashed, no trace has been found there, although objects not linked to it such as shipping containers have been discovered.

Investigators have confirmed that three pieces of debris recovered along western Indian Ocean shorelines belong to MH370.

Other so-called sonar contacts found in the investigation did not "exhibit the characteristics of a typical aircraft debris field" but some had "man-made properties" and needed to be eliminated as from MH370, the government agency leading the search, the Australian Transport Safety Bureau (ATSB), said in a statement.

"The ROV (remotely operated vehicle from Chinese ship Dong Hai Jiu 101) will be tasked to reacquire and investigate, through video imaging, a range of sonar contacts which have been identified during previous deep tow operations," the ATSB said.

"Winter weather conditions have, until now, prevented the safe deployment of the ROV, but now sea states are improving."

The agency added that poor weather conditions during the June-August southern hemisphere winter meant the projected completion date for the hunt had been pushed back from December to January-February next year.

More than 110,000 square kilometres of the 120,000-square-kilometre search area have been scoured so far.

The governments of Australia, Malaysia and China, where most of the passengers were from, have agreed to pull the plug once the search area is fully scoured unless "credible new information" emerges.