Investigators hunting for missing Flight MH370 have widened their ocean floor search to a larger zone and don't know where else to focus if the jet can't be found there, authorities announced Wednesday.
The underwater operation has been "modified" and experts have started searching a larger area of the southern Indian Ocean even before a survey of the original zone has been completed.
Until now, the hunt for Malaysia Airlines Boeing 777 had been focused on a 23,000-square mile patch where investigators said it probably crashed.
The expanded zone covers 46,000 square miles — an area about the size of Pennsylvania — and will be increasingly difficult to cover as the local winter weather sets in.
The jetliner disappeared on March 8 last year en route from Kuala Lumpur to Beijing. A joint investigation by Australia and Malaysia, helped by other international experts, has so far concluded from satellite data that the jet probably changed course and headed south for several hours before running out of fuel.
More than three-quarters of the original search area has been covered so far with no sign of the doomed plane, the Australia Transportation Safety Bureau's Joint Agency Coordination Centre said in a short written update.
"The search into the expanded area has already commenced, with search efforts focused in the south to take advantage of the last of the usable weather in that area," it said. "The search plan has been modified to enable continuous search operations during winter and to ensure that the entire [46,000 square mile] area is searched as quickly and effectively as possible.
The statement added: "Expert advice is that the highest probability of locating the aircraft is within the [46,000 square mile] search area. Beyond that, it is not possible to refine the search area to one of greater likelihood."
The statement did not explain further, and the ATSB did not immediately respond to a request for clarification.
The expanded search zone was agreed in April by ministers from Australia, Malaysia and China. Most of the missing passengers are Chinese.
Three specialist ships involved in the ocean floor search — GO Phoenix, Fugro Discovery and Fugro Equator - were all bound for, or returning from, the Australian supply port of Fremantle.
"Our work will continue to be thorough and methodical, so sometimes weekly progress may seem slow," it said. "Please be assured that work is continuing and is aimed at finding MH370 as quickly as possible."
The expert vessels last week found a previously-uncharted shipwreck almost 13,000 feet below the surface.
This article originally appeared on NBCNews.com.