Donald Trump and French President Emmanuel Macron have moved to build bridges after a diplomatic flare-up threatened to derail Nato talks hosted by Boris Johnson.
The US president earlier denounced a claim by Mr Macron that the alliance was suffering from “brain death” as “very insulting” to other member states.
But when the two men met later at the US ambassador’s residence in London, Mr Trump acknowledged the need for greater “flexibility” in the way in which Nato responded to global threats.
Mr Macron in turn accepted the US had “over-invested” in the alliance for “decade after decade” – a long-standing gripe of the US leader.
Mr Macron’s original comments were prompted by the Turkish offensive against the Kurds in northern Syria – regarded by most Nato members as key allies in the fight against so-called Islamic State (IS).
He complained the US had effectively given the green light for the incursion – withdrawing its troops from the region – without any consultation with European allies, despite the wider consequences.
At a breakfast meeting with Nato secretary general Jens Stoltenberg, Mr Trump said it had been a “very, very nasty” statement by the French leader.
“Nobody needs Nato more than France,” he said. “That’s why I think when France makes a statement like they made about Nato, that’s a very dangerous statement for them to make.”
When they met later, Mr Macron insisted he stood by his original remarks, while acknowledging US concerns that other allies had not borne their fair share of the financial burden for collective defence.
He cited the US unilateral withdrawal from the INF treaty with Russia banning medium range missiles as well as the threat of terrorism as reasons why “strategic clarification” was needed over the way Nato delivered long-term peace in Europe.
“The common enemy today are the terrorist groups. I am sorry to say that we don’t have the same definition of terrorist around the table,” he said.
“When I look at Turkey they are now fighting against those who fought with us against Isis and sometimes they work with Isis proxies.
“It is still an issue and it is a strategic issue. I do believe we need strategic clarification – how to deliver long term peace in Europe.”
Mr Trump said they had had a “very good discussion”, and welcomed Mr Macron’s comments about financial burden-sharing.
“I appreciate you saying the United States for decades has really been paying proportionately way, way too much for Nato and other countries paying far too little,” he said.
He said they had both agreed on the need for more flexibility in the way that Nato operated.
“A lot of people say it was originally meant to look at the Soviet Union, now Russia, but we also have other things to look at whether it is radical Islamic terrorism, whether it is the tremendous growth of China,” he said.
The meeting of Nato leaders was being held to mark the alliance’s 70th anniversary.
Mr Johnson hosted talks with Mr Macron, Turkish President Recep Tayyip Erdogan and German Chancellor Angela Merkel ahead of the main gathering in an attempt to iron out some of the differences between them.
Following the discussions, a Downing Street spokesman said: “The leaders discussed the broad strategic, economic and defence partnerships between their countries and agreed the importance of further deepening these, including through Nato.”
The No 10 spokesman said there was consensus on Syria with the leaders agreeing that “humanitarian access, including cross-border, must be ensured”.
Leaders concurred that a United Nations review should “form the basis for getting aid to those who require it in the north-east” of the war-torn country – the area US troops were withdrawn from suddenly in October, allowing Turkish forces to take on Kurdish militia who had been involved in defeating so-called Islamic State.
The situation in Libya was also discussed.
The meeting came amid concern among European allies that the Turkish offensive in northern Syria had strengthened the position of Russia and of the Assad regime, while triggering another humanitarian crisis.
There was further alarm among alliance members when Mr Erdogan chose to purchase Russian air defence systems – seen as a further weakening of his commitment to the alliance.
The formal Nato events began with a reception at Buckingham Palace hosted by the Queen on Tuesday evening.
The main talks will then take place on Wednesday at The Grove, a country house hotel near Watford.
The meeting is expected to consider new threats, including in the areas of cyber and space, after the alliance last month declared space one of its operational domains alongside air, land, sea and cyber.