President Donald Trump has railed against a deal to curb Iran's nuclear program, but officials say that far from scrapping it, he is considering kicking the decision to Congress.

Ahead of an October 15 deadline, several officials familiar with White House deliberations told AFP Trump has made it clear he does not want to certify Iran's compliance with the accord.

The 2015-era Obama agreement offered Tehran relief from punitive economic sanctions, in return for limits to uranium enrichment and intrusive inspections.

Every 90 days Trump must decide whether the Iran is living up to its end of the bargain, something that has already caused him political pain on two occasions.

The Trump administration has publicly accused Iran of violating the "spirit" of the accord -- known as the JCPOA -- although some officials privately admit there is a thin line between testing the limits and a material breach.

Trump's top military advisor, the chairman of the joint chiefs of staff, General James Dunford, has told Congress the briefings he has received "indicate that Iran is adhering to its JCPOA obligations."

But Republicans are under domestic political pressure to fulfil campaign and donor promises to scrap the accord.

The case against

Trump has called the deal an "embarrassment to the United States" and had urged allies and fellow signatories in London, Paris and Berlin to renegotiate it, something they are unwilling to do.

But now a middle path is being explored, which would make Trump's opposition clear, but stop short of scrapping the deal outright and perhaps clear the 90-day-review off his desk.

Under the plan, Trump could find Iran in breach or -- less provocatively -- refuse to certify Tehran's compliance, giving Congress 60 days to decide whether to impose sanctions.

The issue has prompted fierce debate inside the administration, and with this mercurial president, anything is still possible between now and the deadline.

But "it seems like he was leaning that way," said one official, echoing the accounts of others who refused to speak on the record, because of the sensitivity of the subject.

Hawks are urging confrontation with Iran, while others warn of inflaming the Middle East and seriously damage ties with European allies who consider the agreement in their vital national interest.

Some aides are also warning against escalating tensions at a time when an analogous nuclear stand-off with North Korea worsens.

Yes, senator, I do

The debate appeared to spill out into the open on Tuesday when Defense Secretary Jim Mattis contradicted Trump's claim that the deal "poses a direct national-security threat."

Mattis told the Senate Armed Services Committee that "at this point in time, absent indication to the contrary, it is something that the president should consider staying with."

Asked whether he believed the Iran deal was in the national interest, Mattis replied: "Yes, senator, I do."

In kicking the ball to Congress, Trump could still open the door to rupturing the deal.

Lawmakers could decide to impose sanctions that, if implemented, would shatter the terms of the international agreement and leave the United States in breach.

In July, four influential Republican Senators -- Tom Cotton, David Perdue, Ted Cruz and Marco Rubio -- sent a letter to the administration claiming Iran had broken the accord on four counts.

But behind closed doors, the proposal is being greeted coolly by some Republicans on Capitol Hill, who have an already testy relationship with the outsider president.

A Congressional source said it was "too early to say" whether there would be enough support to pass sanctions.

Much depends on how Trump frames their decision -- if he demands Congress do their job and sanction a "nefarious" Iran, then pressure will grow.

"I can absolutely see why the White House wants to kick it to Congress: the President gets the advantage of not certifying, but Congress has to take responsibility for what that means," said Kori Schake, formerly of the White House for the National Security Council and now the Hoover Institution.

"It seems to be a questionable strategy for the White House to set in motion, where the President gets to be reckless but banks on Congress to act heroically and prevent disaster."

Doing so, she added, requires "an awful lot of trust that the President won't turn around and campaign against them for it."

The White House hopes to use the threat of sanctions to gather support for a harder international line on Iran.

It would like to see curbs on Tehran's ballistic missile program, and end to its support for militias across the Middle East and a revision of "sunset clauses" that would permanently end sanctions without a permanent end to the nuclear program.

They are issues that allies believe should be addressed separately or in a successor agreement. There is also growing concern that if the deal survives, the same issue does not come every 90 days.

Some of the deal's reluctant supporters inside the White House are pressing for the certification requirement to be removed in any congressional action, a move that could also make life easier for Trump.