Australian officials said the objects were spotted by satellite in one of the remotest parts of the globe, around 2,500 km (1,500 miles) southwest of Perth in the vast oceans between Australia, southern Africa and Antarctica.
The larger of the objects measured up to 24 meters (78 ft), and appeared to be awash over water several thousand meters deep, they said.
"It's credible enough to divert the research to this area on the basis it provides a promising lead to what might be wreckage from the debris field," Royal Australian Air Force Air Commodore John McGarry told a news conference in Canberra.
No confirmed wreckage from Malaysia Airlines Flight MH370 has been found since it vanished from air traffic control screens off Malaysia's east coast early on March 8, less than an hour after taking off from Kuala Lumpur for Beijing.
"I can confirm we have a new lead," Malaysian Transport Minister Hishammuddin Hussein told reporters in Kuala Lumpur, where the investigation into the missing airliner is based.
Another official in Malaysia said investigators were "hopeful but cautious" about the Australian discovery.
The fate of Flight MH370 has been baffling aviation experts for nearly two weeks.
Investigators believe that someone with detailed knowledge of both the Boeing 777-200ER and commercial aviation navigation switched off the plane's communications systems before diverting it thousands of miles off its scheduled course.
Exhaustive background checks of the passengers and crew aboard have not yielded anything that might explain why.
The huge potential breakthrough in an investigation that had appeared to be running out of leads was revealed by Australian Prime Minister Tony Abbott, who told parliament the objects had been located with satellite imagery.
"New and credible information has come to light in relation to the search for Malaysia Airlines Flight MH370 in the southern Indian Ocean," Abbott said.
He added that he had already spoken with his Malaysian counterpart, Najib Razak, and cautioned that the objects had yet to be identified.
"The task of locating these objects will be extremely difficult and it may turn out they are not related to the search for MH370," Abbott said.
John Young, general manager of the emergency response division of Australian Maritime Safety Authority (AMSA), told reporters at the Canberra news conference that an Australian air force AP-3C Orion plane was already at the scene, and more aircraft were on the way.
A merchant ship diverted for the task was due to arrive in a few hours, he said. A Royal Australian Navy ship equipped to recover any objects was also en route, but was still "some days away."
"They are objects of a reasonable size and probably awash with water moving up and down over the surface," Young said.
He said it could be some days before authorities have anything to report and added that poor visibility reported in the area could hamper the search.
"It's probably the best lead we have right now but we have to get there, find them, see them, assess them, to know whether it's really meaningful or not," he said.
The dimensions given are consistent with at least one of the objects possibly being the major part of a 777-200ER wing, which is around 27 metres long, though Australian officials cautioned the first images were indistinct.