Trump pushing Iran to make firmer nuclear commitments in initial deal: Sources

President Donald Trump is demanding that Tehran put specific nuclear concessions down in writing as part of a preliminary agreement aimed at pushing past the drawn-out deadlock between the U.S. and Iran, U.S. officials and another source familiar with the matter tell ABC News.

Iranian negotiators previously gave verbal assurances that the regime would ultimately agree to certain terms related to Iran’s nuclear program, but the president determined during a meeting in the Situation Room on Friday that those commitments were not strong enough, the sources said.

During testimony before the Senate Foreign Relations Committee on Tuesday, Secretary of State Marco Rubio shared some detail on what the Trump administration wanted to see from Iran before moving forward.

"They have to commit to very specific negotiations on highly enriched -- the disposition of the highly enriched uranium that still is buried deep in a mountain somewhere," Rubio said. "They have to agree on negotiating severe and long-term limitations and/or cancellation of enrichment activity in their country."

Rubio said the details, as well as financial incentives for Tehran, could be determined later.

"For example, they have to commit to say 'we will dispose of the enriched uranium. And the question now is 'What are the mechanisms by which we do so?' That can be negotiated," he said.

For weeks, talks between the U.S. and Iran have centered on reaching a preliminary memorandum of understanding, which would formally extend the fragile ceasefire between the countries, gradually reopen the Strait of Hormuz and set a timeline for further negotiations on nuclear issues.

Last week, American and Iranian negotiating teams landed on a draft of that deal they said they believed could win the support of both Trump and the Iranian regime. That version included a written commitment from Iran that it would not pursue a nuclear weapon -- a pledge Iran's leaders have also publicly made multiple times in recent years -- but did not include specific promises related to its broader nuclear program, which the regime has always insisted is intended for peaceful purposes.

Iran is reviewing the updated terms, and officials say its negotiators have not indicated whether the regime is likely to accept them.

U.S. officials say Trump is wary of making any major monetary concessions to Iran during this phase of the negotiations. Rubio testified that the administration had not offered to lift sanctions targeting Iran or unfreeze any of its assets as part of the initial deal, insisting that would only be on the table once Iran followed through with promises to curb its nuclear program.

The president told ABC News' Chief Washington Correspondent Jonathan Karl on Monday that he expects Iran will agree to his terms within the week, but U.S. officials have continued to express doubt about fractures within the Iranian regime and its ability to coalesce behind an agreement.

A state-sponsored Iranian news agency reported on Tuesday that the latest draft of the potential deal was still under review in Tehran.

"You ultimately are negotiating with people who then have to negotiate within their own system to see what they're allowed to give and what they're allowed to agree to," Rubio said on Tuesday.

However, Rubio also said that although Iran’s Supreme Leader Mojtaba Khamenei has not been seen or heard from publicly since he was injured in the strike that killed his father during the opening hours of the war, there were now "indications" that he is "increasingly engaging at some level" in negotiations.