John Kerry, the US secretary of state, struggled to persuade a deeply sceptical Congress to abandon a plan for new sanctions against Iran on Monday, warning lawmakers that they risked scuppering a major nuclear agreement.

Kerry said the notion of introducing new sanctions, a month after a historic interim agreement was reached with Tehran, and in the midst of negotiations for a final, comprehensive deal, was "gratuitous".

"I'm not saying never," Kerry told members of the House foreign affairs committee, most of whom favour more sanctions to increase pressure against Iran. "I'm just saying: not right now. Let me be very clear: this is a delicate diplomatic moment."

Kerry's attempts to sell the current policy towards Iran, however, met a wall of opposition from members of the committee, who appeared almost entirely unified against the deal, which some argued had endangered the US and its ally Israel.

A succession of committee members, Republican and Democratic, told Kerry they did not trust Iran's commitments and said the only viable way to avoid Tehran from acquiring a nuclear weapon was to press forward with yet tighter sanctions.

Kerry countered that the world was at "a crossroads, one of those hinge points in history", and said there would be "gigantic implications" if the talks broke down. "They know that if this fails, sanctions will be increased," he said of Iran's leaders. "But you don't need to do it [now]. It is actually gratuitous, in the context of the situation. Because you can do it in a week if you need to."

Under the interim agreement, forged last month between Iran and a six-nation group comprising the US, three European countries, Russia, and China, Tehran has agreed to halt progress of its nuclear program, neutralise a stockpile of higher grade uranium and open its facilities to inspectors.

In return, some sanctions have been eased – which, according to US estimates, will provide a $7bn boost to Iran's debilitated economy. The agreement, brokered in Geneva, will last just six months, after which all sides are seeking to achieve a final, comprehensive deal.

The Republican chairman of the foreign affairs committee, Ed Royce, and the Democratic ranking member, Eliot Engel, authored the bill to increase sanctions against Iran, which passed 400 votes to 20 in July. Both said at the hearing that they had serious concerns about the Geneva agreement, arguing that tightening sanctions on Iran at this stage would actually strengthen the hand of US negotiators.

A major concern for sceptics of the deal is the possibility that Iran will be permitted to maintain a limited, civil nuclear-enrichment program on its soil, rather than import nuclear materials from abroad. "Iran is not just another country," Royce told Kerry. "It simply can’t be trusted with enrichment technology, because verification efforts can never be foolproof."

The Royce-Engel bill, which would dramatically tighten existing sanctions, is currently with the Senate banking committee. But there is growing pressure for the legislation to be put to a vote, possibly with a clause that would mean the implementation would be delayed by six months, allowing the current round of talks to continue.

When Iran’s foreign minister, Javad Zarif, was asked how Tehran would respond in the event of Congress imposing new sanctions with a six-month delay, the usually conciliatory diplomat responded: “The entire deal is dead. We do not like to negotiate under duress." Zarif, Iran's lead negotiator, told Time magazine: "And 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.”

Although most western leaders have cautiously backed the Geneva deal, Israel's prime minister, Binyamin Netanyahu, has called the agreement a "historic mistake", and his officials have been pressing their case hard in Washington.

Kerry looked visibly irritated during his testy exchanges with members of the committee, warning them that any new sanctions jeopardize the achievements already achieved by negotiations and undermine the unity the US has achieved with other powers such as China and Russia. "It implies a lack of faith in the process, and an unwillingness to play by the rules that our partners are playing by," he said.

He insisted there was only "very limited and reversible" easing of sanctions under the interim agreement, saying the $7bn Iran is forecast to gain from the deal "pales in comparison" to the $30bn it will still lose from the many other sanctions that have remained intact. In an effort to win support, Kerry complimented members of Congress and said they should take "great pride" in the fact that years of sanctions had brought about the historic negotiations.

As well as freezing Iran's nuclear program, Kerry said the provisional settlement provided inspectors with daily access to Fordow, the secret mountain-top enrichment plant, as well as unprecedented access to a heavy-water plutonium reactor, Arak. "I think Congress deserves an enormous amount of credit for that," he said.

Few on the committee were persuaded, repeating that Iran could not be trusted and that the provisional agreement falls short of what the US should be demanding.

The debate over the new sanctions bill is merely a foretaste of the major confrontation between Congress and the Obama administration that could be unleashed in July if a comprehensive deal is reached. Although the White House can ease some sanctions through waiver authorities, experts agree that unraveling the wider sanctions regime, which stretches back three decades, would require congressional approval.

The challenge of securing that support from Congress was laid bare by Kerry's reception from the committee. Obama's critics are not restricted to Republicans, but include hawks from the president's own Democratic party. In the Senate, they include the chairman powerful foreign relations committee, Robert Menendez of New Jersey, and the third-ranking Democrat, Chuck Schumer of New York.

Obama insisted on Saturday that regardless of the outcome of talks, the "first-step" deal forged in Geneva had at least postponed Iran's so-called “breakout capacity”. However, the president said that the likelihood of a satisfactory outcome from the negotiations with Iran was no more than “50-50”.