Secretary of State John Kerry said Tuesday that a full accounting of Iran's possible past atomic weapons research is not necessarily critical to reaching a nuclear deal with Tehran. His comments came amid concerns the Obama administration is backing down on demands that Iran resolve concerns about previous work as part of an agreement that would curb its nuclear program in exchange for sanctions relief.

Kerry said the U.S. and its negotiating partners are "not fixated" on the issue of so-called "possible military dimensions" because they already have a complete picture of Iran's past activities. He said they are more concerned that those activities have stopped and about what Iran might do in the future. Negotiators are concentrating their efforts on that as they race to meet a June 30 deadline to reach a deal, Kerry said.

"The possible military dimensions, frankly, gets distorted a little bit in some of the discussions in that we're not fixated on Iran specifically accounting for what they did at one point in time or another," he told reporters at the State Department by video link from his home in Boston, where he is recovering from surgery on a broken leg.

"We know what they did," Kerry said. "We have no doubt. We have absolute knowledge with respect to certain military activities they were engaged in. What we're concerned about is going forward. It's critical to us to know that going forward, those activities have been stopped and that we can account for that in a legitimate way."

"That clearly is one of the requirements, in our judgment, for what has to be achieved in order to have a legitimate agreement" he said. "And in order to have an agreement, to trigger any kind of material, significant sanctions relief, we would have to have those answers."

After reaching an interim accord with Iran in November 2013, the Obama administration said a comprehensive solution "would include resolution of questions concerning the possible military dimension of Iran's nuclear program."

But officials told The Associated Press last week that those questions won't be answered by the June 30 deadline. Instead, they said, the U.S. and its partners are working on a list of future commitments that Iran must fulfill to resolve the concerns about past work.

Much of Iran's alleged work on warheads, delivery systems and detonators predates 2003, when Iran's nuclear activity first came to light. But Western intelligence agencies say they don't know the extent of Iran's activities or if Iran persisted in covert efforts. An International Atomic Energy Agency investigation has been foiled for more than a decade by Iranian refusals to allow monitors to visit suspicious sites or interview individuals allegedly involved in secret weapons development.

Iran denies any work on or interest in nuclear weapons, insisting that its enrichment of uranium, plans for a heavy water reactor and various research activities are meant for power generation and other peaceful applications. It says pieces of evidence cited by the IAEA and others are forgeries, and officials including Ayatollah Ali Khamenei have made several defiant statements about blocking monitors from accessing Iranian facilities and scientists.

Critics of the emerging Iran deal have focused significantly on the issue of Iran's past military work. They insist Iran must not only "come clean" on such activity for transparency's sake, as past and present U.S. administrations have long demanded, but that compliance with any accord can only be measured if Tehran provides a complete accounting of all its previous nuclear efforts. Critics contend that otherwise, the world wouldn't have a full script of everything it needs to verify.

On Monday, the chairman of the Senate Foreign Relations Committee, Sen. Bob Corker, R-Tenn., expressed alarm about reports of concessions, particularly "a less-than-full disclosure of possible military dimensions of Tehran's nuclear program."

"I hope reports indicating potential concessions on inspections and on the full disclosure of Iran's possible military dimensions are inaccurate," he wrote.