Military radar showed the plane turned back across Peninsular Malaysia after contact with it was lost. A handful of faint "pings" picked up by a commercial satellite for around another six hours helped narrow down its likely final location.
Martin Dolan, Chief Commissioner of the Australian Transport Safety Bureau which is leading the search, said he remained confident that the plane would be found in the remainder of the so-called "priority search area".
If, however, the search has to be expanded into the much larger surrounding area, the costs could prove prohibitive.
"It's almost impossible to get your head around the scale of what's involved here," he told Reuters.
"If you take the theoretical maximum of the possible area for the aircraft – 1.1 (or) 1.2 million sq km – you're talking about orders or magnitude in terms of cost and time above what we're currently doing, and that's something that governments will obviously have to bear in mind."
Most of those on board the lost flight were Chinese or Malaysian.
Chinese Foreign Ministry spokesman Hong Lei said Australia, China and Malaysia had cooperated closely on the search.
"The Australian side has put in a large amount of personnel and material resources and we are deeply grateful for their help," Hong said. "The search effort is still ongoing and we hope the relevant work will produce progress."
The Malaysian government did not immediately respond to requests for comment.
Loss-making Malaysia Airlines, whose fortunes worsened when another of its Boeing 777's was shot down over Ukraine on July 17, killing all 298 people on board, was delisted at the end of 2014 as part of a $1.8 billion government-led restructuring.
Rounding out a bad year for Malaysian-affiliated carriers, an Indonesia AirAsia flight from Surabaya to Singapore crashed on Dec. 28, killing all 162 people on board.