* New southern search area a 600,000 sq km corridor
* AMSA says search remains "needle in a haystack"
* Search also continues along northern arc
* Rough seas and remote location to hamper search
By Jane Wardell and Lincoln Feast
SYDNEY, March 18 (Reuters) - Australia's maritime safety agency drastically narrowed the search area in the southern Indian Ocean for the missing Malaysia Airlines jetliner on Tuesday, but said the remote location, high seas and swift currents meant the task remained daunting.
The Australian Maritime Safety Authority (AMSA) shrunk its search field to a 600,000 sq km (230,000 sq mile) corridor, just 3 percent of the estimated 19 million sq km area in the Indian Ocean where the plane could be, based on satellite tracking data.
Still, the revised area is roughly the size of Spain and Portugal combined and will take the Australian-led southern search team several weeks to comb.
"A needle in a haystack remains a good analogy," John Young, general manager of the emergency response division of AMSA, told reporters. "The aircraft could have gone north or south and if it went south, this is AMSA's best estimate of where we should look with the few resources we have at our disposal for such a search."
AMSA said its revision of the search area was based on analysis of satellite data collected from the plane by the United States National Transportation Safety Board (NTSB) it received on Monday.
AMSA then streamlined that data further to account for water movements and changes in weather in the days since Flight MH370 disappeared 10 days ago.
"It's the result of some analysis of the possible movement of the aircraft," Young said. "There are some assumptions built in, including the speed of the aircraft."
The original search area for flight MH370 focused on a wide strip of territory either side of two arcs formed by satellite plots of the aircraft's last known possible position, an area measuring 38 million sq km.
The northern hemisphere search area is along an arc stretching from Malaysia through northern Thailand, Myanmar and China to Kazakhstan. Young said he had no details on whether that search area has been similarly streamlined.
Australia took charge of the "southern vector" of the search for the plane that had 239 people on board on Monday at the request of Malaysian Prime Minister Najib Razak.
The southern Indian Ocean is one of the most remote places in the world and also one of the deepest, posing enormous challenges.
Seas in the search area characterised by by 3-metre waves and winds of 25 knots, Young said.
Adding to the difficulty, the revised search area is about 1,500 nautical miles (2,800 km) southwest of Perth, meaning aircraft have limited time to search the area once they reach it from the mainland.
"Our purpose first is to find anyone alive, if there is anyone alive, and secondly, prove or discount any possibility that the aircraft came south," Young said.
Two Australian AP-3C Royal Australian Air Force Orion aircraft already searching the area will be joined on Wednesday with another two Australian Orions, a Royal New Zealand Air Force P3-K2 Orion and a U.S. Navy P-8 Poseiden. China has offered assistance and was in discussion with Australia about joining the search team, Young said.
Ships in the southern Indian Ocean have been requested to keep a lookout and to travel through the search area if possible. One ship is currently in the area, with another due to travel through on Wednesday.
Young said that Australia will attempt to refine the search area still further if more data becomes available.