Macron has criticised Trump's "American First" nationalism that has shaken the transatlantic alliance.

The smiles and warm handshakes were back between Donald Trump and his French counterpart Emmanuel Macron on Thursday as they met in northern France, a sharp turnaround from a bad-tempered meeting last November.

The two leaders took centre stage on the 75th anniversary of the D-Day landings in World War II when 150,000 Allied troops began the liberation of Nazi-occupied France.

Both used the occasion to recall their countries shared history and sacrifice as observers scrutinised their body language for signs of any lingering ill-will following their clashes last year.

There were repeated handshakes, while Trump greeted Macron's wife Brigitte with kisses on three separate occasions during the main commemoration event at a US cemetery in Colleville-sur-Mer where more than 9,000 US servicemen are buried.

"It's been good sometimes and sometimes it hasn't been, but right now it's outstanding," Trump said later of his relationship with Macron as they sat down for a working lunch in the nearby town of Caen.

"So the relationship that we've had together has been really terrific and I appreciate it very much," Trump added.

The last time Macron hosted Trump in France -- for the World War I centenary in November last year -- it turned into a diplomatic fiasco.

Trump arrived angry about Macron's support for a European army and he mocked the French leader on Twitter several days afterwards over his "very low approval ratings".

A French diplomat said that the US president had been angered by a speech during the WWI commemorations and had also resented not being given a bigger role in the gathering that brought 70 leaders together.

In his WWI speech, Macron had said that "nationalism is a betrayal of patriotism" in a 20-minute address at the Arc de Triomphe that also criticised "saying our interests come first and others don't matter."

Again on Thursday, he delivered several thinly-veiled messages to Trump about his "America First" nationalism and isolationist tendencies that have roiled the transatlantic alliance.

"America is never as big as when it is fighting for the freedom of others," he said.

He added that "we must never stop working for the alliance of the free world" while citing the UN, NATO and EU as multilateral organisations that stemmed from World War II, but which are regularly criticised by Trump.

'Inappropriate' criticism?

The French diplomat said that after the open hostilities in November, Macron and Trump had held several phone calls during which they rebuilt a relationship that had started remarkably well after Macron's election in 2017.

"Our approach has stayed the same: we continue to try to persuade and at the same time to cushion the impact when we haven't succeeded," said the diplomat, who was speaking on condition of anonymity.

The problem for Macron is that he has had few successes in persuading Trump to change his mind, while the policy disagreements have mounted.

The two leaders are at odds over Iran, the role of the EU, climate change and trade, while they hold fundamentally different views on how states and allies should interact with each other.

"Macron is not shy about saying the problem in the world is the populist nationalist movement," Trump's one-time adviser and campaign manager Steve Bannon told AFP in a recent interview.

"Macron is always looking to take a shot at the nationalists and I think sometimes he's done it in inappropriate situations," he added.

Unpopular in France

Where the two men have repeatedly found common ground is over military action.

Their armies are in action against the Islamic State group in Syria and Iraq and they struck the regime of Syrian President Bashar al-Assad in April 2018 in retaliation for an alleged chemical attack against civilians.

"We've had great success working together too," Trump said in his comments with Macron. "Your military is excellent. My people report back, they say it's absolutely excellent. They work very well together."

Macron is aware that being critical of the American president plays well domestically in France where Trump is unpopular.

A poll by the YouGov survey group released on Wednesday showed that only 17 percent of French people had a positive view of the former reality TV star.

And only 24 percent thought Macron should take a more cooperative approach with him.