US President Donald Trump appeared to leave a window open for diplomacy with Iran on Monday after French President Emmanuel Macron flew in Iran's Foreign Minister to a G7 summit in an effort to reduce US-Iranian tensions.

European leaders have struggled to tamp down the brewing confrontation between Iran and the US since Trump pulled Washington out of world ­powers' 2015 nuclear deal with Tehran and ­reimposed ­sanctions on the Iranian ­economy.

But Macron has spent the summer trying to create ­conditions for a period of pause to bring the two sides backs to the negotiating table.

Those efforts took a surprise turn on Sunday when Iranian Foreign Minister Mohammad Javad Zarif, who is under US sanctions, flew to the French seaside town of Biarritz where the Group of Seven leaders were meeting.

Zarif held talks with Macron and British and German officials before returning home. 

Though potentially a diplomatic minefield, Macron's gamble with Zarif appears to have worked out for now, as Trump on Monday endorsed the French president's initiative and toned down his usual harsh rhetoric on Tehran.

While Trump reaffirmed Washington's goal of extracting farther-reaching security concessions from Iran, he told reporters at the summit he wanted to see "a really good Iran, really strong, we're not looking for regime change."

"I knew [Zarif] was coming in and I respected the fact that he was coming in. We're looking to make Iran rich again, let them be rich, let them do well, if they want," Trump said.

The 2015 deal between Iran and six world powers, reached when Trump's predecessor Barack Obama was in office, aimed to curb Iran's disputed uranium enrichment program in exchange for the lifting of many international sanctions on Tehran.

Since ditching the deal last year, Trump has pursued a policy of "maximum pressure" to try to force Iran into broader talks to restrict Iran's ballistic missile program and end its support for proxy forces around the Middle East as well.

While Trump's European allies also want fresh negotiations with Iran, they believe the nuclear deal must be upheld to help ward off the risk of wider war in the Middle East. Macron had already met Zarif in Paris on Friday ahead of the G7.

"What we want is very simple. It's got to be non-nuclear [as well]," Trump said. 

"We're going to talk about ballistic missiles... about the timing. But they (Iran) have to stop terrorism. I think they are going to change, I really do."

He said that he had not wanted to meet Zarif himself, because it was too soon.

The Iranian leadership also appeared upbeat. "Road ahead is difficult. But worth trying," Zarif said in a tweet.

"Those who imagine one hand is enough are mistaken. We have to use our military, cultural and economic power as well as political power, diplomacy and negotiation," President Hassan Rouhani said on his official website.