* Objects spotted in extremely remote location
* Australian PM says search planes investigating
* Norwegian merchant vessel arrives in search area (Updates with comment from New Zealand air force)
By Jane Wardell and Siva Govindasamy
SYDNEY/KUALA LUMPUR, March 20 (Reuters) - Aircraft and ships ploughed through dire weather in search of objects floating in remote seas off Australia that Malaysia's government called a "credible lead" in the trans-continental hunt for a jetliner missing for nearly two weeks.
New Zealand's air force, whose P-3K2 Orion returned from a 2 1/2-hour search mission in the Indian Ocean early on Friday, said it had found nothing that could have come from the Malaysia Airlines Boeing 777 which disappeared on March 8.
"The crew never found any object of significance," Kevin Short, air vice marshal at New Zealand's Defence Forces, told Radio New Zealand.
"Visibility wasn't very good, which makes it harder to search the surface of the water," he said, adding that the plane had flown at around 1,000 feet (300 metres) over the sea.
The large objects which Australian officials said were spotted by satellite four days ago are the most promising find in days as searchers scour a vast area for the plane lost with 239 people on board.
A Norwegian merchant ship arrived in the area on Thursday, but officials cautioned it could take days to confirm if the objects were part of the Malaysia Airlines Boeing 777. Malaysia's government said the search would continue elsewhere despite the sighting in the southern Indian Ocean.
The area where the objects were spotted is around 2,500 km (1,500 miles) southwest of Perth, roughly corresponding to the far end of a southern track that investigators calculated the aircraft could have taken after it was diverted.
"Yesterday I said that we wanted to reduce the area of the search. We now have a credible lead," Malaysian Transport Minister Hishammuddin Hussein told reporters in Kuala Lumpur.
A search for the plane that began in the tropical waters off Malaysia's east coast has now switched to the vast, icy southern oceans between Australia, southern Africa and Antarctica.
In addition to the New Zealand craft, two Royal Australia Air Force AP-3C Orions and a U.S. Navy P-8 Poseidon were involved in Thursday's search, which was called off late in the evening and will resume on Friday.
There have been many false leads and no confirmed wreckage found from Flight MH370 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.
Hishammuddin said the information on the objects received from Australia had been "corroborated to a certain extent" by other satellites, making it more credible than previous leads.
The larger of the objects measured up to 24 metres (79 ft), long and appeared to be floating in water several thousand metres deep, Australian officials said. The second object was about five metres (16 feet) long. Arrows on the images pointed to two indistinct objects apparently bobbing in the water.
"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.
The satellite images, provided by U.S. company DigitalGlobe , were taken on March 16, meaning that the possible debris could by now have drifted far from the original site.
Australian officials said an aircraft had dropped a series of marker buoys in the area, which will provide information about currents to assist in calculating the latest location.
The captain of the first Australian Air Force AP-3C Orion plane to return from the search area described the weather conditions as "extremely bad" with rough seas and high winds.
A Norwegian car carrier diverted from its journey from Madagascar to Melbourne and had arrived in the search area, the ship's owner said. A Royal Australian Navy ship equipped to recover any objects was also en route.
China's icebreaker for Antarctic research, Xuelong, or Snow Dragon, will set off from Perth to search the area, Chinese state news agency Xinhua cited maritime authorities as saying. About two-thirds of the 227 passengers on Flight MH370 were Chinese nationals.
Chinese President Xi Jinping told Australian Prime Minister Tony Abbott in a telephone call that he hoped Australia would do all it could to search the area and offer assistance to Chinese search ships, China's Xinhua news agency reported.
"At present, search and rescue work is quite difficult, and the situation in the relevant seas is complex. As long as there is a thread of hope, we must put in 100 percent effort," Xinhua quoted Xi as saying.
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 yielded barely anything that might explain why.
The discovery of the floating objects was revealed by Abbott.
"The task of locating these objects will be extremely difficult and it may turn out they are not related to the search for MH370," he told parliament.
The dimensions of the objects given are consistent with at least one of them possibly being the major part of a 777-200ER wing, which is around 27 metres (89 feet) long, though Australian officials cautioned the first images were indistinct.
The relatively large size of the objects would suggest that, if they do come from the missing aircraft, it was largely intact when it went into the water.
If the plane had run out of fuel, it would not necessarily have plummeted but its behaviour would have depended on whether there was someone in control and their intentions, pilots said.
Modern aircraft are designed to use the rush of wind to drive a small emergency turbine that keeps hydraulics and some electrical power running if the engines run out of fuel.
If the debris is from the plane, investigators would face a daunting task to retrieve the "black box" data and voice recorders needed to help understand what caused the disaster.
University of Western Australia Professor of Oceanography Charitha Pattiaratchi said that, based on currents in the area, if the debris is from the plane it probably would have entered the water around 300-400 km (180-250 miles) to the west.
The search area covered an ocean ridge known as Naturalist Plateau, a large sea shelf about 3,500 metres (9,800 feet) deep, Pattiaratchi said. The plateau is about 250 km (150 miles) wide by 400 km (250 miles) long, and the area around it is close to 5,000 metres (16,400 feet) deep.
"Whichever way you go, it's deep," Pattiaratchi said.
Investigators piecing together patchy data from military radar and satellites believe that, minutes after its identifying transponder was switched off as it crossed the Gulf of Thailand, the plane turned sharply west, re-crossing the Malay Peninsula and following an established route towards India.
What happened next is unclear, but faint electronic "pings" picked up by one commercial satellite suggest the aircraft flew on for at least six hours. That would be consistent with the plane ending up in the southern Indian Ocean.
The methodical shutdown of the communications systems, together with the fact that the plane appeared to be following a planned course after turning back, has focused particular attention on the pilot and co-pilot.
The FBI is helping Malaysian authorities analyse data from a flight simulator belonging to the captain of the missing plane, after initial examination showed some data logs had been deleted early last month.
A Malaysian official with knowledge of the investigations into the pilots said three simulator games that 53-year-old pilot Zaharie Ahmad Shah had played were being looked at.
"We are following up on the data logs being erased," the source said. "These could be logs of the games that were erased to free up memory, so it may not lead us to anything. He played a lot of games, going into hundreds and thousands of hours." (Additional reporting by Tim Hepher, A. Ananthalakshmi, Anuradha Raghu and Niluksi Koswanage in Kuala Lumpur, Byron Kaye and Lincoln Feast in Sydney, Neil Darby in Perth and Mark Hosenball in Washington, Naomi Tajitsu in Wellington; Writing by Alex Richardson and Stuart Grudgings; Editing by Nick Macfie and Tom Brown)