The underwater search area for Malaysia Airlines Flight MH370 has been expanded in the southern Indian Ocean, the Joint Agency Coordination Centre (JACC) said Wednesday. So far, search vessels have scoured over 75 percent of the seafloor in the initial search zone with no concrete clues as to the whereabouts of the missing plane.

The search plan for the Boeing 777-200 has been “modified” to conduct a continuous search for the jetliner in a bid to locate the missing plane. However, search operations in the expanded zone will be increasingly difficult to cover as winter weather sets in, forcing the search vessels to focus in the south “to take advantage of the last of the usable weather in that area.”

“Expert advice is that the highest probability of locating the aircraft is within the 120,000 square kilometer [46,000-square-miles] search area.  Beyond that, it is not possible to refine the search area to one of greater likelihood,” JACC said, in its latest search update.

Officials also said that the search in the expanded area of about 46,000 square miles has already commenced, adding that the safety of the search crew was also a priority. Last month, officials from Australia, China and Malaysia, agreed that the search area would be doubled if the aircraft was not found by May. The initial search had focused on a 23,166-square-mile portion of the southern Indian Ocean. The plane went missing on March 8, 2014, with 239 people on board, while on its way from Kuala Lumpur to Beijing. 

Four vessels -- Fugro Discovery, Fugro Equator, Fugro Supporter and GO Phoenix -- are involved in the current search operation in the priority search zone, located about 1,100 miles off the western coast of Australia.

Authorities remain optimistic about locating the plane despite months of extensive search for the plane having unearthed no trace of the passenger jet. The international effort has become the costliest search operation in aviation history.

“In the event the aircraft is found and accessible, Australia, Malaysia and the People's Republic of China have agreed to plans for recovery activities including securing all the evidence necessary for the accident investigation,” JACC said, in its statement.