The Iran nuclear deal is not intended to push Tehran to reform but prevent it building a bomb, US Secretary of State John Kerry has told a sceptical Congress.

In his second appearance on Capitol Hill in a week, America's top diplomat stressed on Tuesday that the plan reached this month with Tehran was the best deal achievable.

The suggestion by his Republican critics that a tougher US negotiating stance could have forced a complete climb-down by the Iranians was, he argued, naive wishful thinking.

"Let me underscore: the alternative to the deal that we have reached is not some kind of unicorn fantasy that contemplates Iran's complete capitulation," Kerry warned before the House Foreign Affairs Committee.

Rejecting the deal - as Congress has the power to do - would essentially give Iran a green light to return full-speed to its enrichment efforts.

"It's clear. If Congress rejects this, Iran goes back to its enrichment, the ayatollah (Iran's supreme leader Ayatollah Ali Khamenei) will not come back to the table and Iran will say, 'We're free. We can go about, back to our program,'" Kerry said.

Scepticism has soared among US representatives since Washington and five world powers reached an historic accord with Iran that would rein in the Islamic republic's nuclear program in exchange for an easing of the sanctions that have crippled Iran's economy.

Last week, senior Republican senators accused Kerry of being "fleeced" by Tehran during hard-fought negotiations. Several Republicans, including many running for president in 2016, advocate walking away from the deal.

Amid the debate, the American public has wavered. A CNN/ORC poll released on Tuesday revealed 52 per cent of respondents say Congress should reject the deal, while 44 per cent say it should be approved.

That contrasts with some major polls in recent weeks showing most Americans supportive of the agreement.

With Congress having a crucial say, the White House has launched a full-court press to win over enough members to prevent the accord from collapsing.

And while some have criticised the deal because it does not oblige Iran to recognise Israel or require Tehran to renounce support for terrorist activities, Kerry said it was vital Congress recognise that the deal focuses purely on nuclear weapons.

"This plan was designed to address the nuclear issue alone, not to reform Iran's regime, or end its support for terrorism, or its contributions to sectarian violence in the Middle East," Kerry told the panel during a four-hour grilling.

Washington "will continue to push back against Iran on every front available", Kerry said.
"But the fact is it's a lot easier to push back against an Iran that doesn't have a nuclear weapon than one that does."