Peace negotiators are pressing Iran to make a concession on its nuclear program to revive ceasefire talks with the U.S. after President Trump rejected Tehran’s truce proposal, in a race to stop the conflict from escalating back into all-out war.

Missiles are displayed next to a banner of Iran's supreme leader, Mojtaba Khamenei, in Tehran.

Mediators involved in the talks said it is a long-shot bid to get Iran to placate Trump on the issue he cares about most. Trump has tried to reframe the conflict around his original war goal of preventing Iran from obtaining a nuclear weapon, and redirect talks away from unlocking the Strait of Hormuz from Iran’s grip. In a speech to the United Nations last week, Trump said he was weighing whether to “annihilate” Iran’s government to keep it from gaining a nuclear bomb.

But Iran has given mediators little indication that it is ready to talk about its nuclear program.

Iranian leaders are confident that they can withstand a long-term U.S. blockade or a renewed bombing campaign from the Americans, said Middle East government officials who speak with Tehran regularly. The officials said Iranian power is now concentrated in the hands of the Islamic Revolutionary Guard Corps, a paramilitary and economic force that has long taken a hard line against compromise with the West.

On Sunday, Iranian Foreign Minister Abbas Araghchi said Iran was open to negotiations but said Tehran would respond to any aggression in kind.

“We are fully prepared for the war to resume,” Araghchi said on NBC’s “Meet the Press.” “We stand firm in the face of any new aggression, even if it comes to a doomsday war. But at the same time we stand ready for diplomacy. It is up to President Trump to choose.”

Araghchi stayed in New York over the weekend after the General Assembly for additional talks, but Arab mediators said Iran hasn’t compromised at all since Trump rejected Tehran’s ceasefire plan.

Arab officials in the Persian Gulf said Iran wants Washington to offer concessions before negotiations can move forward.

Trump himself is skeptical that diplomacy will work and has told his staff he sees a renewed bombing campaign as likely, The Wall Street Journal reported. On Friday, Trump rejected an Iranian ceasefire proposal that would have opened the Strait of Hormuz and ended the U.S. blockade of Iranian ports for seven days to give space for negotiations.

The U.S. has become more confident that its naval blockade is having a severe effect on the Iranian economy. At the same time, U.S. and Gulf officials said Iran is losing control of the Strait of Hormuz, with millions of barrels of oil passing through an essential waterway for crude-oil transit and the global economy.

“This weakens Iran’s leverage on the negotiating table,” said Umer Karim, a researcher on Gulf security at the U.K.’s University of Birmingham.

Karim said Iran’s increasingly hard-line government believes Trump and his Republican Party will lose the midterm elections. From there, many Middle East officials believe Trump will be less constrained by electoral politics and will be more willing to use force, and less likely to make a deal on Iran’s terms.

“Right now, a concession at this time by any party would be considered an expression of weakness by the other and not just a willingness to settle matters,” Karim said.

Iran wants the U.S. to return to a memorandum of understanding that Trump agreed to in June that would have reopened the Strait of Hormuz and eventually launched nuclear negotiations in exchange for the lifting of some sanctions on Iran, the unfreezing of billions of dollars of Tehran’s money held overseas, and the lifting of the U.S. blockade, among other matters.

That agreement was widely seen as favorable to Iran, and Trump no longer wants to return to it, U.S. officials have said. Instead, the U.S. has tightened sanctions on Iran and any country doing business with it, including trying to stop air traffic in and out of the Islamic Republic.

“It should be no surprise that all sides are hardening their positions,” said Behnam Ben Taleblu, the senior director of the Iran program at the Foundation for Defense of Democracies, a Washington think tank that is critical of Tehran’s policies. “Tehran is trying to backend Trump into a compressed version of the agreement it violated earlier this summer. Meanwhile Washington appears intent on escalating sanctions and maintaining the blockade until it senses surrender.”

A high-level official with a Middle East government involved in peace talks said Iran would be willing to make compromises on its nuclear program and was close to doing so earlier this year in talks with Vice President JD Vance. However, the official said Iran would need to be given explicit assurances that Israel wouldn’t attack again after any agreement between Washington and Tehran.

Write to Summer Said at summer.said@wsj.com, Benoit Faucon at benoit.faucon@wsj.com and Gavin Bade at gavin.bade@wsj.com