The latest round of talks, led by U.S. Trade Representative Michael Froman, has been continuing since Wednesday. But the outlook for a deal was uncertain. A USTR web page on Sunday night said a final press conference had been "delayed pending further notice."
The TPP is a central focus of President Obama's trade agenda and something he has pushed for since the start of his presidency. Any agreement among the 12 countries will have to be OK'd by Congress.
The countries taking part -- Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the U.S. and Vietnam -- together make up 40% of the world's economy.
The U.S. and Japan -- the first and third largest economies in the world -- have anchored the talks.
China, the No. 2 economy, is not part of the TPP. Proponents hope an agreement could neutralize some of China's power in global trade.
Trade deals are always political and business lightning rods because they create winners and losers. The North American Free Trade Agreement, put in place in 1994, still sparks fierce debate.
The TPP is no different.
One contentious issue has been how long pharmaceutical companies should be allowed to control information about the medicine they develop. The U.S. is pushing for longer terms, but other countries like Australia want shorter ones.
Drug companies say longer periods of exclusivity over research encourages investment in developing cures. But longer terms can also keep cheaper versions off the market. The potential for compromise over this issue is reportedly a final key to a deal.