WASHINGTON — The Obama administration and Congress clashed Tuesday over the historic 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, Secretary of State John 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. … 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 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.

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 that Iran is adhering to the interim pact, according to legislative aides.

Without that certification, the legislation would reimpose 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.

Iran’s foreign minister, Mohammad Javad Zarif, has warned that 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 U.S. 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.”

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.

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.