Leaders of the Senate Foreign Relations Committee said Thursday they will press the U.N.'s nuclear watchdog to share information on how it will account for Iran's past work that many believe was aimed at producing a nuclear weapon.

The "roadmap" worked out July 14 between Iran and the International Atomic Energy Agency for resolving questions about past nuclear work has become a major sticking point in the congressional review of the nuclear deal. Congress is reviewing the deal and is likely to hold some vote on whether to approve or disapprove it.

At a Thursday hearing, Foreign Relations Chairman Bob Corker announced that IAEA Director General Yukiya Amano refused a request to appear before lawmakers next week to discuss the issue. Agency officials had previously refused to share the actual roadmap or any of its annexes, which Secretary of State John Kerry and other U.S. officials also say they have not seen.

Corker, a Republican from Tennessee, said he was preparing a letter for committee members to sign that will ask Amano to reconsider.

"From the beginning, it's been our hope that we can get direct communications with the IAEA," said Ben Cardin of Maryland, the committee's ranking Democrat. "We'll continue to press for that."

The IAEA's acceptance of Iran's explanations is a condition for the lifting of international sanctions under the agreement. Nonpartisan nuclear experts have noted that Iran's full accounting of any past nuclear weapons work is essential to establishing a baseline for international inspectors to verify compliance with terms of the agreement limiting Tehran's nuclear program.

In a series of hearings over the past two weeks, Kerry and other officials have said the White House is satisfied with the IAEA's process and have offered to brief lawmakers on the roadmap. But that has proven not to be enough as the controversy has festered.

Lawmakers from both parties insist they must know the details of how those questions will be resolved before they vote on the deal, which must be done before the Oct. 15 deadline for Iran to respond to the U.N. agency.

"I am deeply, deeply disappointed in their unwillingness to come in any session, public or classified, to have a discussion," Sen. Bob Menendez, D-N.J., said Thursday of the IAEA.

"The entire inspection regime, the entire verification regime depends upon the IAEA," he said. "I don't know how one can come to a conclusion on this agreement without understanding from the agency that is involved — the most critical element of this agreement is them."