US President Donald Trump is expected to announce tonight that he will decertify the Iran nuclear deal — the landmark pact made in 2015 to curb the country's nuclear program.

He has until October 15 to make a decision, but is expected to outline the administration's new approach to Iran in a speech at the White House at 3:45am on Saturday (AEDT).

As we wait for an announcement, let's take a look at what the deal involves, why Mr Trump isn't a fan, and what could happen if he decides to walk away.

What's the Iran deal again?

In 2015, Iran agreed to restrict its nuclear program in return for the US, the European Union and United Nations lifting most of the sanctions which had crippled its economy.

The deal was signed by the United States, Britain, France, Germany, Russia, China, the European Union and Iran.

It was a major win for both former-US president Barack Obama and Iran's president Hassan Rouhani, who was elected on the promise of reducing Iran's diplomatic isolation.

Opponents say the deal went too far in easing sanctions without requiring that Iran end its nuclear program permanently.

But supporters say its collapse could trigger a regional arms race and worsen Middle East tensions.

What does Trump think of the deal?

Both Mr Trump and his administration have frequently criticised the policy.

In his speech to the UN General Assembly in September, Mr Trump described it as "one of the worst and most one-sided transactions the United States has ever entered into".

"We cannot abide by an agreement if it provides cover for the eventual construction of a nuclear program," he said.

"Frankly, that deal is an embarrassment to the United States."

Mr Trump addressed 193 diplomats at the UN General Assembly's New York headquarters. ( AP: Seth Wenig )

What's he said recently?

Last week Mr Trump doubled down on that criticism during a meeting with military leaders at the White House.

"We must not allow Iran … to obtain nuclear weapons," he said.

"The Iranian regime supports terrorism and exports violence, bloodshed and chaos across the Middle East.

"That is why we must put an end to Iran's continued aggression and nuclear ambitions. They have not lived up to the spirit of their agreement."

So what DOES Trump want?

A senior administration official, speaking on condition of anonymity, told the Reuters news agency that Mr Trump was expected to decertify the deal and roll out a broader US strategy on Iran that would be more confrontational.

Mr Trump says Tehran is in violation of the spirit of the agreement and is currently considering a strategy that could allow more aggressive US responses to Iran's forces, its Shiite Muslim proxies in Iraq and Syria, and its support for militant groups.

A State Department official said the Trump administration ultimately wanted to "bring about a change in the Iranian regime's behaviour".

When asked to elaborate, the official said that behaviour included:

Ballistic missiles proliferation

Ballistic missiles proliferation Support for terrorism

Support for terrorism Support for Syrian President Bashar al-Assad

Support for Syrian President Bashar al-Assad Unrelenting hostility to Israel

Unrelenting hostility to Israel Consistently threatening freedom of navigation in the Persian Gulf

Consistently threatening freedom of navigation in the Persian Gulf Cyber attacks against the United States and its allies

Cyber attacks against the United States and its allies Human rights abuses; and

Human rights abuses; and Arbitrary detentions of US citizens

So, is Iran complying with the deal?

The International Atomic Energy Agency says Iran is complying with the agreement.

That's despite Mr Trump arguing that it has done nothing to rein in its ballistic missile program or its financial and military support for Hezbollah and other extremist groups.

Do Republicans agree with Trump?

Many of Mr Trump's fellow Republicans, who control Congress, have also been critical of the Iran nuclear deal.

But a US official said that like the heated rhetoric with North Korea on its nuclear program, the Iran discussion has vexed White House Chief of Staff John Kelly, Defense Secretary Jim Mattis and Secretary of State Rex Tillerson.

Each of them "have tried to advise the President that there are significant risks in the course he'd prefer to pursue".

It follows Defence Secretary Jim Mattis comments at a congressional hearing two weeks ago that Iran was "fundamentally" in compliance with the agreement.

General Mattis believes the US should stay in the deal. ( Reuters: Francois Lenoir )

He said the US should consider staying in the deal unless it were proven that Tehran was not abiding by it or that it was not in the US national interest to do so.

When General Mattis was asked by a senator whether he thought staying in the deal was in the US national security interest, he replied: "Yes, senator, I do".

What do others say?

CIA Director Mike Pompeo said on Thursday that Tehran is "mounting a ruthless drive to be the hegemonic power in the region".

But the prospect that Washington could renege on the pact has worried some of the US allies that helped negotiate it.

Europe has warned of a split with the US on the issue, prompting British Prime Minister Theresa May and French President Emmanuel Macron to appeal to Mr Trump for the US to certify the deal for the sake of allied unity.

"It's imperative that Europe sticks together on this issue," German Foreign Minister Sigmar Gabriel told the RND German newspaper group.

"We also have to tell the Americans that their behaviour on the Iran issue will drive us Europeans into a common position with Russia and China against the USA."

Mr Macron has previously said there was no alternative to the nuclear accord, formally known as the Joint Comprehensive Plan of Action (JCPOA).

A senior Iranian diplomat told Reuters the end result of Mr Trump's expected move would be to isolate the United States since the Europeans would continue to support it.

"Many foreign investors told us that they will not be scared away from Iran's market if Trump decertifies the deal," the diplomat said.

Israeli Prime Minister Benjamin Netanyahu, a close ally of Mr Trump, last month said that unless provisions in the accord removing restrictions on Iran's nuclear program over time are eliminated, it should be cancelled.

"Fix it, or nix it," Mr Netanyahu said in his speech at the UN General Assembly annual gathering of world leaders in September.

Israeli Prime Minister Benjamin Netanyahu says "fix it or nix it". ( AP: Pablo Martinez Monsivais )

So what will happen if Trump does decertify the deal?

If Mr Trump does decline to certify Iran's compliance, it would not mean a US withdrawal from the deal.

Instead, it would give US congressional leaders 60 days to decide whether to reimpose sanctions on Tehran suspended under the agreement.

Whether Congress would be willing to reimpose those sanctions is far from clear.

While Republicans, and some Democrats, opposed the deal when it was approved in 2015, there is little obvious appetite in Congress for dealing with the Iran issue now.

Last month, Iran's foreign minister said Tehran may abandon the deal if Washington decides to withdraw.

ABC/Reuters