French President Emmanuel Macron hosted his US counterpart Donald Trump for a previously unscheduled lunch Saturday ahead of the official opening of the G7 summit in southwest France.

The two leaders sat on a terrace at the ornate Hotel du Palais in the Atlantic resort of Biarritz, the G7 venue, just a day after Trump reiterated his threat of tariffs against French wine in retaliation for a new tax on major US tech companies.

The surprise lunch appeared to be an attempt by Macron to get the summit off on a good footing after a tense build-up in which Trump and EU leaders traded threats of trade war.

Speaking to reporters in fluent English, Macron called Trump "a very special guest".

He said that they would be discussing a raft of pressing international issues, including the standoff with Iran over its alleged nuclear weapon ambitions, and stressed that "strong coordination" was needed.

Trump met that appeal for unity with a somewhat softer approach of his own, having left the United States on Friday with a dire warning about taxing French wines "like they've never seen before".

"We actually have a lot in common," Trump said across the table to Macron, adding that they had "been friends for a long time."

Trump said that at times they "go at each other a little bit" but had a "special relationship".

The setting for the G7 summit in the French seaside resort clearly pleased the US president.

"So far so good. The weather is perfect. Everybody's getting along. I think we will accomplish a lot this weekend," he said.