Negotiators from 12 Pacific countries concluded a week of talks in Brunei on a free-trade agreement on Friday but announced no breakthroughs in discussions that one official called "difficult".
The effort pushed by US President Barack Obama to create an Asia-Pacific free-trade area covering nearly 40 percent of global economic output has run into turbulence amid protectionist reflexes, casting doubt on hopes of concluding the pact by year-end.
"Trans-Pacific Partnership (TPP) negotiators intensified their work this week to close gaps between them... to discuss possible landing zones on remaining sensitive and challenging issues," a joint statement said, giving no substantive details.
But a Malaysian official said negotiators made little progress.
"I know it was a difficult round," the senior Malaysian trade official told AFP, providing no specifics.
The talks have been shrouded in secrecy through 19 rounds since 2010, hosted by the countries involved.
The TPP has stirred protests in various nations amid fears it could leave domestic markets exposed to foreign competition.
Washington wants negotiations completed this year.
"I don't think it is a realistic timeline," the Malaysian official said, adding that the country's government was yet to decide whether it would ultimately remain in the effort.
"We have reached a critical stage. So now we need to assess, to take stock -- what if we continue, what if we don't."
Powerful agriculture lobbies in Japan are resisting the TPP and concerns have been raised that Japanese demands for exceptions may present a sticking point.
"There was no sector that did not make any progress (in Brunei)," Koji Tsuruoka, Japan's chief TPP negotiator, told a news conference in the sultanate Friday.
"On the other hand, there was no sector that has been resolved and completed," he added.
Malaysian Trade Minister Mustapa Mohamed last week said his country had "serious difficulties" with the potential impact on state-owned firms.
The TPP joint statement said further meetings were expected in the coming weeks but gave no specifics.
However, Japanese media said a gathering of top negotiators was being arranged for September 18-21 in Washington.
Delegates have previously expressed hopes of concluding the pact in time for a major economic summit in October.
The annual summit of the 21-economy Asia-Pacific Economic Cooperation (APEC) bloc is held this year in Bali, Indonesia.
APEC includes all 12 TPP countries -- Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States and Vietnam.