WASHINGTON -- The American Israel Public Affairs Committee, the largest pro-Israel lobby in Washington, issued a rare statement on Wednesday raising concern with recent remarks made by US Secretary of State John Kerry on a key US position in nuclear talks with Iran.

Via teleconference on Tuesday, Kerry said his negotiators are not "fixated" on forcing Iran to account for all of its past experimentation with nuclear weaponry. The international community, he says, already knows what Iran attempted to construct in over two decades building its nuclear program.

"We’re not fixated on Iran specifically accounting for what they did at one point in time or another.  We know what they did. We have no doubt," he said. "What we’re concerned about is going forward."

The response to his comments has been swift, prompting the secretary's new spokesman, John Kirby, to open his first briefing with an attempt to "put a fork" in the notion that the US was conceding on a key negotiating position.

"The [International Atomic Energy Agency]'s concerns about possible military dimensions, past and present, have to be resolved before there is a deal," Kirby told the press, charging that it is "absolutely, completely false" that the US has shifted on the matter.

The IAEA has sought access for its investigation into Iranian PMD (possible military dimensions) for several years. That, in part, requires access to sites categorized as military installations by the Islamic Republic.

Iran says it will reject all requests for access to its military sites, citing its need to protect "state secrets" and its sovereignty as a UN-member state.

But "sanctions lifting is only going to occur when those steps have been taken," Kirby asserted.

Since his initial comments, Kerry has made several calls to parties concerned with his comments, including to Senator Lindsey Graham (R-South Carolina), a presidential candidate and frequent critic of the Obama administration's foreign policy.

AIPAC's statement was one of concern that, after sealing a framework to a nuclear deal with Iran, the US was "backtracking" on a fundamental negotiating position.

"The secretary’s statement indicates the US may be backtracking from previous demands that Iran respond fully to the IAEA’s questions– raising the disturbing prospect that the anticipated agreement will be fundamentally flawed," AIPAC's statement reads.

The lobbying group argues that a clear outline of Iran's past military nuclear work is necessary in establishing "a baseline of Tehran’s nuclear program– including breakout time– against which to measure future actions."

"The US and the IAEA do not know the extent of Iran’s nuclear capabilities," they state. "When asked in April if Iran must disclose past military-related nuclear activities as part of an agreement, Secretary Kerry said 'They have to do it. It will be done. If there’s going to be a deal, it will be done.'"

Phasing of sanctions relief is a significant challenge left for negotiators to resolve since a framework for a nuclear deal was agreed upon in Lausanne, Switzerland, back on April 2.

Reporters and other interested parties surrounding the Iran talks are unclear on whether the US wants a resolution to the PMD issue to precede a deal itself, or to precede sanctions relief.

Also unclear is whether a full resolution to the IAEA's concerns would precede all sanctions relief, or merely some of that relief.

"It is very clear what the expectations are of Iran," Kirby asserted, noting that the issue has figured "prominently" in the final round of negotiations.

Diplomats from the US, Russia, China, France, Britain, Germany and Iran hope to clinch a comprehensive nuclear agreement by June 30.