WASHINGTON — Secretary of State John Kerry clashed with congressmen on both sides of the aisle Tuesday over the controversial nuclear deal with Iran, exposing deep rifts over a U.S. pledge to refrain from any new sanctions over the next six months in exchange for concessions on enriching uranium.

The disagreement could have broad consequences for the U.S. diplomatic effort to prevent Iran from developing nuclear weapons.

In his first congressional testimony since last month's Geneva agreement,  Kerry defended the diplomacy as having halted and rolled back central elements of Iran's nuclear program for the first time.

He pleaded with Democrats and Republicans alike not to scuttle the chances of a peaceful resolution to a crisis that has regularly featured U.S. and Israeli threats of potential military action.

"Let me be very clear: This is a very delicate diplomatic moment and we have a chance to address peacefully one of the most pressing national security concerns that the world faces today," Kerry told the House Foreign Affairs Committee. "We're at a crossroads. 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."

Kerry's appearance came as lawmakers increasingly threatened to undermine the six-month interim pact, which gives Iran $7 billion in sanctions relief over the next half-year in exchange for the Islamic republic's neutralizing its higher-enriched uranium stockpiles, not adding any new centrifuges, and ceasing work at a heavy-water reactor that potentially could produce plutonium used in nuclear weapons.

Members of both parties challenged Kerry.

The top Democrat on the panel, Rep. Eliot Engel of New York, specifically asked Kerry why the administration was so strongly opposing sanctions that wouldn't be imposed unless Iran breaks the agreement.

And Democratic Rep. Brad Sherman expressed misgivings about trusting the Obama administration, which he accused of hampering all sanctions efforts against Iran thus far.

Sens. Bob Menendez, D-N.J., and Mark Kirk, R-Ill., are close to completing a bill that would require the administration to certify every 30 days Iran's adherence to the interim pact, according to legislative aides.

Without that certification, the legislation would re-impose all sanctions and introduce new restrictions on Iran's engineering, mining and construction industries.

The legislation also calls for a global boycott of Iranian oil by 2015 if Iran fails to live up to the interim agreement. Foreign companies and banks violating the bans would be barred from doing business in the United States.

However, Iran sanctions were left off a defense bill working its way through the Senate this week — much to the dismay of Republicans.

"This is a rather transparent attempt to prevent a vote on enhanced Iran sanctions, so they're trying to circumvent the Senate, pass major legislation, essentially without amendments," Senate Minority Leader Mitch McConnell, R-Ky., told reporters.

In the House, Majority Leader Eric Cantor, R-Va., is drafting separate legislation mapping out how a final deal with Iran should look, aides say.

Iran's foreign minister, Mohammad Javad Zarif, has warned any new package of commercial restrictions would kill the deal.

"If Congress adopts sanctions, it shows lack of seriousness and lack of a desire to achieve a resolution on the part of the United States," Zarif told Time magazine. "My parliament can also adopt various legislation that can go into effect if negotiations fail. But if we start doing that, I don't think that we will be getting anywhere."

Kerry said new sanctions could also be viewed as a sign of bad faith by America's negotiating partners — Britain, France, Germany, China, and Russia. The United States is banking on them to enforce existing oil and financial restrictions on Tehran and to press Iran into a final agreement.

"I don't want to give the Iranians a public excuse to flout the agreement," Kerry said. "It could lead our international partners to think that we're not an honest broker, and that we didn't mean it when we said that sanctions were not an end in and of themselves but a tool to pressure the Iranians into a diplomatic solution. Well, we're in that. And six months will fly by so fast, my friends, that before you know it, we're either going to know which end of this we're at or not."

Kerry's assessment comes just three days after President Barack Obama began to play down chances for success, telling a think-tank forum that he believed the odds of a comprehensive nuclear agreement next year are 50-50 or worse.

Still, Obama defended diplomacy as the best way to prevent Tehran from acquiring atomic weapons and rejected criticism from Israel's government and many in Congress that his aides bargained away too much without securing a complete halt to Iran's nuclear program — as demanded by the international community for several years.

Members of Congress generally believe that crippling petroleum, banking, and trade sanctions levied on Iran in recent years were responsible for bringing its more moderate president, Hassan Rouhani, to power and his representatives to the negotiating table.

Many argue more pressure, not less, could break Iran's will and secure better terms in a final agreement.

At several points, Kerry and lawmakers talked over each other as they argued about whether the deal recognized Iran's "right" to enrich uranium — which the administration rejects — and about the details of international inspections on Iranian sites and its non-nuclear weapons programs.

Rep. Ileana Ros-Lehtinen, R-Fla., was perhaps strongest in her criticism of the administration, flatly denouncing the agreement in Geneva as a "bad deal."

"We may have bargained away our fundamental position," said Rep. Ed Royce, R-Calif., the committee chairman. "Iran should not be enriching and reprocessing," he said, criticizing what he termed the administration's "false confidence that we can effectively check Iran's misuse of these key nuclear bomb-making technologies."

Iran insists its program is solely for peaceful nuclear energy and medical research.