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The "jumbo Iran negotiation" sought by US President Donald Trump will be "very, very difficult" to achieve, British Foreign Secretary Boris Johnson warned Monday.

US counterpart Mike Pompeo on Monday set 12 tough conditions for a "new deal" with Iran after Trump walked away from the existing nuclear agreement.

But Johnson said that he didn't see a comprehensive deal "being very easy to achieve, in anything like a reasonable timetable."

Britain and the other powers who signed the 2015 agreement have vowed to honour the deal, despite the US promising to increase financial pressure on Iran with the "strongest sanctions in history," hampering companies hoping to deal with the Middle Eastern state.

Johnson defended the original deal, saying it "protected the world from an Iranian nuclear bomb, and in return it gave the Iranians some recognisable economic benefits."

He added that G20 foreign ministers would discuss the issue when they meet in Buenos Aires later Monday.

"I think we will be discussing, not in the session but we'll certainly be discussing it with friends and colleagues today, how to take it forward," he told reporters.

"But the prospect of a new jumbo Iran treaty is going to be very, very difficult.

"I think in the end, we will get back to the kind of additions to the JCPOA that we initially envisaged - but it may take a long time," he added.

Johnson said Britain was already taking measures to try to shield British companies dealing with Iran from being hit by US sanctions.

"We're going to do everything we possibly can," he said.