Top Iranian and U.S. negotiators resumed talks on Sunday to address disputes over Tehran's nuclear program, in a push for progress as Washington hardens its stance ahead of U.S. President Donald Trump's Middle East visit.

Though Tehran and Washington both have said they prefer diplomacy to resolve the decades-long dispute, they remain deeply divided on several red lines that negotiators will have to circumvent to reach a new nuclear deal and avert future military action.

Iranian Foreign Minister Abbas Araqchi and Trump's Middle East envoy Steve Witkoff will hold the fourth round of talks in Muscat through Omani mediators, despite Washington taking a tough stance in public that Iranian officials said would not help the negotiations.

Araqchi, before departing for Muscat, told Iranian state TV that "Iran has well-known positions based on clear principles... We hope to reach a decisive stance in Sunday's meeting." He added that Iran's expert team was in Oman and "will be consulted if necessary."

Witkoff told Breitbart News on Thursday that Washington's red line is: "No enrichment. That means dismantlement, no weaponization,” requiring the complete dismantling of Iran’s nuclear facilities in Natanz, Fordow, and Isfahan.

"If they are not productive on Sunday, then they won't continue and we'll have to take a different route,” Witkoff said about the talks.

Trump, who has threatened military action against Iran if diplomacy fails, will travel to Saudi Arabia, Qatar and the United Arab Emirates on May 13-16.

Reacting to Witkoff's comments, Araqchi said on Saturday that Iran would not compromise on its nuclear rights, which includes uranium enrichment.

Tehran is willing to negotiate some curbs on its nuclear work in return for the lifting of sanctions, according to Iranian officials, but ending its enrichment program or surrendering its enriched uranium stockpile are among "Iran's red lines that could not be compromised" in the talks.

A senior Iranian official close to the negotiating team said that U.S. demands for "zero enrichment and dismantling Iran's nuclear sites would not help in progressing the negotiations."

"What the U.S. says publicly differs from what is said in negotiations," the official said, on condition of anonymity.

He said matters would become clearer at the talks on Sunday, initially planned for May 3 in Rome but postponed due to what Oman described as "logistical reasons."

Moreover, Iran has flatly ruled out negotiating its ballistic missile program and the clerical establishment demands watertight guarantees Trump would not again ditch a nuclear pact.

Trump, who has restored a "maximum pressure" campaign on Tehran since February, exited Tehran's 2015 nuclear pact with six world powers in 2018 during his first term and reimposed tough sanctions that have devastated Iran's economy.

Iran, which has long said its nuclear program is peaceful, has breached the 2015 pact's nuclear curbs since 2019, including "dramatically" accelerating its enrichment of uranium to up to 60% purity, close to the roughly 90% level that is weapons-grade, according to the U.N. nuclear watchdog.