(Bloomberg) -- Diplomatic attempts to revive a nuclear deal between Iran and world powers are entering their final act, according to European negotiators, who cautioned that political leaders still need to take difficult decisions to strike an agreement. 

Almost 10 months after talks intended to restore the 2015 agreement began, envoys in Vienna adjourned their eighth round of discussions. Energy markets are closely watching for progress that would allow the Persian Gulf nation, with the world’s No. 2 gas and No. 4 oil reserves, to return to international markets. 

“January has been the most intensive period of these talks to date,” negotiators from France, Germany and the U.K. said in an emailed statement. “We are reaching the final stage, which requires political decisions.” 

The adjournment followed another four tough weeks of talks in the Austrian capital. U.S. officials have shown increasing signs of frustration at the pace of the negotiations to revive an accord that lifted sanctions on the Islamic Republic in return for strict limits on its nuclear program. The European Union’s coordinator of the talks, Enrique Mora, also suggested it’s time for leadership in capitals to make decisions that can advance the process.

Russia’s chief negotiator said this week that a deal looked “quite realistic” by the end of February. “The talks are already at an advanced stage,” Michael Ulyanov wrote in a statement via Twitter on Thursday. 

Diplomats are expected to reconvene next week.

Sides have been bogged down for months over Iran’s demand that the U.S. provide guarantees it won’t again abandon the agreement, as well as questions over the fate of uranium-enrichment capacity operated by the country’s engineers. 

U.S. Secretary of State Antony Blinken said this week that negotiations cannot be allowed to drag on too much longer. 

In an apparent effort to keep the talks alive, Iran’s foreign minister and a top security official dangled the prospect earlier this week of direct talks with the U.S. if Tehran senses that a “good” deal is within reach. Negotiations have so far taken place through the Europeans, Russians and Chinese, who remain party to agreement. 

After the Trump administration left the accord in 2018, Iran responded by sharply expanding its nuclear enrichment activity beyond what was allowed under the deal, shortening the time it would need to produce atomic warheads. 

The standoff fueled conflicts in the Middle East and spurred a series of attacks on shipping in waterways critical to the global economy. 

Iran has always denied its nuclear program is intended for military uses. 

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