Testifying before the House Foreign Affairs Committee, Kerry said the "first step" deal brokered between Iran and a group of six western nations including the United States halts Iran's enrichment program and in some instances rolls back what progress Iran has made toward manufacturing weapons-grade uranium that could be used to create a nuclear weapon.
Some U.S. and Israeli lawmakers have said the first-step deal is too lenient and gives up sanctions that were effective in crippling the regime for its nuclear ambitions. Kerry stressed the deal reached is good for both sides.
"The national security of the United States is stronger under this first step agreement than it was before," Kerry said. "Israel's national security is stronger than it was the day before we entered into this agreement. And the gulf and Middle East interests are more secure than they were the day before we entered this agreement."
Part of the deal, Kerry, said, includes Iran giving up its full stockpile of 20 percent enriched uranium -- the highest level next to weapons grade.
"So if you remember when [Israeli Prime Minister Binyamin Netanyahu] held up that cartoon at the U.N. with the bomb in it in 2012, he showed the world a chart that highlighted the type of uranium that he was most concerned about," Kerry said. "And he was talking about that 20 percent stockpile. Under this agreement, Iran will forfeit all, not part, all of that 20 percent, that 200-kilogram stockpile. Gone."
Rep. Eliot L. Engel, D-N.Y., the committee's ranking Democrat, expressed reservations Iran would pull a bait-and-switch.
"[The Iranian regime] has ... a history of deceiving the international community about its nuclear program. ... It has that history, and it's pursuing a ballistic missile program in violation of U.N. Security Council resolutions as well," Engel said. "So Iran is not just another country. It simply can't be trusted with enrichment technology, because verification efforts can never be foolproof."
Kerry acknowledged much remains to be done to see whether Iran is truly committed to ending its nuclear enrichment program.
"I came away from our preliminary negotiations with serious questions about whether or not they're ready and willing to make some of the choices that have to be made, but that's what we put to test over the next months," he said.
Still, Kerry said, the two sides are engaged in delicate and complicated diplomatic negotiations -- ones that could result in a normalization of relations if they succeed or set the stage for future conflict if they fail.
"We're at a crossroads," Kerry said. "We're at one of those really hinge points in history. One path could lead to an enduring resolution in the international community's concerns about Iran's nuclear program. The other path could lead to continued hostility and potentially to conflict. And I don't have to tell you that these are high stakes."
He also called on lawmakers to hold off on legislation that would increase sanctions on Iran while negotiators continue to work toward a comprehensive deal.
"We're asking you to give our negotiators and our experts the time and the space to do their jobs. And that includes asking you, while we negotiate, that you hold off imposing new sanctions," he said. "Now, I'm not saying never. I just told you a few minutes ago, if this doesn't work, we're coming back and asking you for more. I'm just saying not right now. Let me be very clear, this is a very delicate diplomatic moment."