The head of Iran’s Revolutionary Guards said Wednesday that the U.S. would experience “painful responses” after President Donald Trump and his top diplomat threatened to walk away from the historic nuclear deal.

The future of the landmark 2015 pact hangs in the balance after Trump’s feisty “America First” speech before the United Nations General Assembly on Tuesday, in which he labeled Tehran a “murderous regime” and warned that the U.S. “cannot abide by an agreement if it provides cover for the eventual construction of a nuclear program.”

Trump described the deal, the centerpiece foreign policy achievement of the Obama administration, as “one of the worst and most one-sided transactions the United States has ever entered into.” The president’s remarks left fellow signatories to the nuclear pact frantically lobbying Washington to stick to the terms of the agreement.

Despite these efforts, Secretary of State Rex Tillerson later told Fox News that the terms had to be changed or the deal would be scrapped, singling out the so-called sunset clauses – under which some curbs on Iran’s nuclear program are incrementally lifted from 2025 – as the main issue. Critics of the deal believe that when those restrictions expire in 10-15 years, Iran would be able to swiftly develop a nuclear weapon.

“If we’re going to stick with the Iran deal, there has to be changes made to it. The sunset provision simply is not a sensible way forward,” Tillerson said.

Under the 2015 deal, brokered with Iran by the U.S., Russia, China, Britain, Germany, and France, Tehran agreed to scale back its nuclear program in return for the easing of sanctions.

The Trump administration has long hinted it would adopt a tougher policy on Iran, which it says has been acting contrary to the spirit of the deal. Washington recently imposed new economic sanctions on the country after it developed missiles – activity that falls outside the scope of the nuclear agreement – and Trump has indicated that he might not certify that Tehran is in compliance with the deal when the deal comes up for renewal next month.

French President Emmanuel Macron responded to Trump’s comments with a strong defense of the nuclear deal, although he indicated a desire to address concerns about the sunset clauses. The French leader called the deal “the best possible agreement available” and warned that renouncing it “would be a grave error. Not respecting it would be irresponsible.”

Acknowledging American concerns about the sunset clauses, he said the best way to address them would be to hold talks on extending the deal’s restrictions on Iran’s nuclear activities beyond 2025.

Sanam Vakil, an Iran expert at Britain’s Chatham House think tank, told VICE News that significant lobbying was now underway as the other signatories to the deal attempted to convince American diplomats not to walk away from the agreement, warning them that “an American decertification of the deal would have devastating consequences.”

“The U.S. is assuming that by playing hardball, it can get a better deal,” Vakil said. But he added that the political climate in Iran, where hard-liners would be angered by Trump’s aggressive rhetoric, meant that it would be difficult for Iranian President Hassan Rouhani to participate in another round of negotiations or make further concessions.

“It’s very risky,” Vakil said. “If Trump backs out of the deal, it will put Rouhani in a very difficult position. It could result in more accelerated conflict.”

Trump’s comments drew a predictably angry response from Iran Tuesday, with Foreign Minister Mohammad Javad Zarif labeling them “ignorant hate speech” that was “unworthy of a reply.” Rouhani, who will address the U.N. Wednesday morning, said in an NBC interview Tuesday prior to Trump’s U.N. appearance that if Washington walked away from the deal, “no one will trust America again.”