A key player in Congress' foreign policymaking posture pressed Secretary of State John Kerry Wednesday about sensitive nuclear talks with Iran, saying Tehran is stonewalling international inspectors about its past work on an atomic bomb.

California Republican Rep. Ed Royce, chairman of the House Foreign Affairs Committee, told Kerry at a hearing that members of the panel have serious concerns about the direction of the talks.

"I'm hearing less about dismantlement and more about the performance of Iran's nuclear program," Royce told Kerry. "That's particularly disturbing when you consider that international inspectors report that Iran has still not revealed its past bomb work."

The secretary testified in the House two days after returning to Washington from the latest round of talks in Geneva involving Iran, the U.S. and five other world powers. U.S. and Iranian officials reported progress on getting to a deal that would clamp down on Tehran's nuclear activities for at least 10 years but would then slowly ease restrictions.

Republican and Democratic lawmakers are skeptical that Iran is negotiating in good faith and accuse Tehran of buying time and meddling throughout the Mideast. Still, a comprehensive pact could ease 35 years of U.S-Iranian enmity — and seems within reach for the first time in more than a decade of negotiations.

Royce said the U.N's International Atomic Energy Agency has expressed concerns about the scope of Iranian military-related activities, including its work in designing a nuclear payload for a missile.

"The IAEA inspectors have amassed over a 1,000 pages which showed research, development and testing activity on technologies needed to develop a nuclear weapon," the congressman said. "Of the 12 sets of questions that the IAEA has been seeking since 2011, Iran has answered part of one of them. ... They are withholding that information."

Kerry agreed that Royce's questions were legitimate and that Iran must answer them if it wants to have an agreement with the U.S. and its partners on a deal to curb its nuclear program.

"They are legitimate and the questions have to be answered," Kerry said. "And they will be if they want to have an agreement."

He said Iran has complied with all the provisions of a first-step agreement, which launched the talks. "They agreed to roll back their program," Kerry said. "I think that's cause for hope."

Wednesday was Kerry's second appearance before Congress in as many days. As he did on Tuesday in the Senate, Kerry told members of the House Foreign Relations Committee that it's inappropriate to condemn what is in an agreement before anybody knows what it is — or even if there even will be a deal.

"We certainly don't think it's appropriate to condemn it before everybody knows what it, in fact, is — if there is an 'is,' " Kerry said.

Negotiators are rushing to try to meet a March 31 deadline for a framework agreement that would keep Tehran from being able to develop nuclear weapons.