World powers adjourned talks in Vienna with plans to return next week, as differences between Iran and the U.S. over how to revive a landmark nuclear deal continue to delay the Islamic Republic’s return to oil markets.

Both the U.S. and Iran will need to make “hard decisions” that could ruffle domestic political constituencies, Enrique Mora, the European Union’s deputy foreign policy chief, said after Wednesday’s talks concluded. Envoys were departing Vienna to return to their capitals, where leaders will have to decide whether the landmark agreement capping Iran’s nuclear work in exchange for sanctions relief is worth reviving.

“There are a number of technical issues that are rather complex, but I can say there are fewer than there were one week ago,” Mora, a Spanish diplomat, told journalists after the fifth round of talks. “The sixth round will definitely be the last.”

The 2015 nuclear deal has been on the verge of collapse since it was abandoned by former President Donald Trump three years ago. His sanctions on Iran’s economy, and the Islamic Republic’s subsequent decision to significantly expand its nuclear program, triggered a crisis in the Persian Gulf that has roiled markets and almost led to war.

A bleak assessment of Iran’s nuclear transparency by International Atomic Energy Agency inspectors cast a pall over this week’s round of talks. 

Monitors circulated two confidential reports raising new questions about Iran’s willingness to address concerns over its past and present nuclear activities in the absence of a return to the deal.

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