The Sydney Morning Herald - Foreign Affairs Minister Julie Bishop has said there is a "powerful argument" that US President Donald Trump's threat to scrap the Iran nuclear deal could imperil efforts to negotiate a peaceful outcome with North Korea.

In a significant sharpening of Australia's public rhetoric on the Obama-era Iran agreement, Ms Bishop said she had asked US officials, including Secretary of State Rex Tillerson, how they would counter the argument that North Korea could not trust the US if it walked away from previous international agreements.

Ms Bishop also said she had a "working assumption" that China was ultimately prepared to cut off North Korea's oil to force it into negotiations to stop its nuclear program.

The US President vowed during his election campaign to tear up the deal, but this month kicked the issue to the US Congress, effectively demanding it either come up with a tougher approach or he would follow through on his threat to scrap it.

By endorsing the argument that ditching the deal could set back efforts to negotiate with North Korea over its nuclear program, Ms Bishop has taken the firm, if diplomatically sensitive position, of pressuring the White House not to push ahead with what many international experts say would be a self-defeating move but one in which Mr Trump is personally invested.

Ms Bishop said she had discussed the matter with American officials last month after Iran mounted the argument to the United Nations General Assembly that the US would lose credibility including in its stand-off with North Korea.

"Iran was immediately on the front foot saying ... 'Why would North Korea sit down and negotiate with the United States as we, Iran did, if the United States feels that it's able to walk away from an agreement that was embodied in a UN Security Council resolution?'" Ms Bishop told Fairfax Media.


"The United States is aware of the argument that Iran was promoting and the currency that it was gaining. It's a powerful argument.

She said that during a visit to Washington immediately after the UN meeting she had asked officials including Mr Tillerson: "How do you counter Iran's argument if you do pull out of the [deal]?"

She said, referring to Congress, that "those on Capitol Hill get the logic".

Asked whether Mr Tillerson got the logic, Ms Bishop replied: "He's a very, very, very logical man."

Mr Tillerson was reportedly among the cabinet members who successfully pressured Mr Trump to hand the deal to Congress rather than simply walk away from it.

The deal, formally called the Joint Comprehensive Plan of Action, was signed under former president Barack Obama and also included Britain, France, Germany, China and Russia. Iran had a range of international economic sanctions lifted in return for stopping production of weapons-grade fissile material for 15 years.

International inspectors and US intelligence agencies say Iran is sticking to the letter of the deal, but Mr Trump has argued that Tehran was breaching the spirit, including by testing ballistic missiles.

Ms Bishop acknowledged the deal was not perfect but said: "Don't unpick the JCPOA for the purposes of trying to negotiate a better deal. Leave that one in place ... and pursue other issues via another channel."

Ms Bishop has previously expressed Australia's opposition to scrapping the deal but her remarks about North Korea significantly strengthen her position. The other five countries involved have also demanded Mr Trump stick with the agreement.

Ms Bishop also indicated she believed China was ultimately prepared to cut North Korea's oil – widely seen as the ace that would force Kim Jong-un to negotiate.

"The answer is oil ... China knows that the [North Korean] army can only last so long, the country can only operate for so long ... They know that's the ultimate key to it."

Asked whether the conclusion therefore was that China would ultimately pull that card, she said: "That's a working assumption."

She said Mr Tillerson's reassurances that Washington did not seek regime change or collapse in North Korea, would not move US military forces north of the border and not pursuing an accelerated reunification of the Korean peninsula had given Beijing the room to be more helpful.

"China's paranoia about the United States is assuaged by Tillerson saying, 'That's not what we're doing China, so you fix it.' It's really changed the debate," she said.

Ms Bishop also said she would be "disappointed" if – as has been reported in the US – Mr Trump does not attend the East Asia Summit as part of his coming Asia trip. But she said if Mr Trump skipped the important meeting, Mr Tillerson would likely attend.