Donald Trump has lashed out at Emmanuel Macron on the first morning of a two-day Nato meeting, saying the French president’s description of Nato as brain dead was insulting and a “very, very nasty statement”.

At a news conference alongside the Nato secretary general, Jens Stoltenberg, the US president also accused Macron of trying to break away from Nato, as well as running a failing economy – while discarding the fact he himself has described Nato as obsolete on previous occasions.

Quick Guide What is Nato and why are its members meeting in London? Show What is Nato? The North Atlantic Treaty Organisation (Nato) is a collective defence alliance between 29 North American and European countries. Founded in 1949, the treaty provides that if one country is attacked, all Nato members would collectively respond. Nato was set up to counter the threat of the Soviet Union. The 12 founding members were: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, United Kingdom and the United States. Over the years the organisation has expanded to its current membership of 29. Bosnia and Herzegovina, Georgia, North Macedonia and Ukraine are recognised as states with aspirations to join. Why is it meeting in London? This week's summit marks a celebration of the 70th anniversary of the organisation. London was chosen partly because it was the location of the original headquarters of the organisation when it was founded. What is on the agenda? During the two-day gathering there will be addresses by the Nato secretary general, Jens Stoltenberg, a formal reception at Buckingham Palace hosted by the Queen, and a meeting of the North Atlantic Council attended by heads of state and government. The agenda features discussions about Russia, China and the future of arms control. There will also be a series of bilateral meetings between leaders - the most testing of which are likely to feature Turkey's Recep Tayyip Erdoğan looking for support for his country's recent incursion into Syria. What's the context? Infighting is a major issue. For the third summit in a row, Donald Trump is expected to renew demands that European allies and Canada step up defence spending. He is also unhappy with his French counterpart, Emmanuel Macron, over a tax on American tech giants including Google, Amazon and Facebook. For his part Macron has lamented Nato's "brain death" due to a lack of US leadership, and said the organisation needs a wake-up call. He insists that strategic questions must be addressed, like improving ties with Russia and how to handle an unpredictable ally like Turkey. In turn, Erdoğan has lashed out at Macron. Their very public argument bodes ill for the summit, which is being hosted by the British prime minister, Boris Johnson. Johnson will want who to smooth things over and downplay any links to Trump, who is unpopular in the UK.

Macron had tried to galvanise the agenda for the summit in London by calling the 70-year western alliance “brain dead”, but Trump said: “Nato serves a great purpose. I think that’s very insulting.”

He added: “Nobody needs Nato more than France. It’s a very dangerous statement for them to make.”

Macron made his criticism of Nato in an Economist interview partly to reflect his frustration that Turkey, a Nato member, had entered northern Syria in October without coordination with any Nato partner apart from Trump. Macron believes the invasion has undermined the fight against Islamic State.

But Trump appeared to side with the Turkish leader, Recep Tayyip Erdoğan, saying Turkey “couldn’t be nicer, more supportive, very helpful.” He added that Turkey had cooperated in the killing of the Isis leader, Abu Bakr al-Baghdadi.

Defending his decision to pull US troops out of north-east Syria, and to green-light the Turkish invasion, he said: “I wanted to get our soldiers out of there, but I wanted to keep the oil.”

He also reeled off a string of insults against France, saying: “I think they have a very high unemployment rate in France. France is not doing well economically at all.

“It is a very tough statement to make when you have such difficulty in France, when you look at what is going on with the yellow vests [anti-government protesters].

“They have had a very rough year. You just can’t go around making statements like that about Nato. It is very disrespectful. I’m looking at him [Macron] and I’m saying that he needs protection more than anybody, and I see him breaking off [from Nato]. So I’m a little surprised at that.”

Asked whether the US alliance with Nato was shaky, Trump denied it, but said: “I do see France breaking off ... I see him breaking off.”

The US and French presidents later gave a joint press conference where relations seemed notably warmer.

A couple of hours after the breakfast, Stoltenberg said that he did not expect alliance members to agree to a common position on Turkey, and in particular to agree to Ankara’s demand that Nato agrees to designate the Syrian Kurdish YPG as a terrorist organisation.

But he insisted that the growing list of divisions would not split the alliance, saying “we’ve always been able to unite around core tasks” and that it would not be possible for Turkey to block Nato operations in the Baltic region. “We are already deployed in the Baltic region,” the Nato leader added.

Earlier, Trump had defended Stoltenberg, boasting that Nato members had massively increased their defence spending thanks to his pressure – but then reiterated his complaints about European spending.

“When I came in, I was angry at Nato, and now I’ve raised $130bn,” Trump said, referring to the sum Stoltenberg says Canada and European members will have added to their defence budgets by next year. “And yet you still have many delinquent – you know, I call them delinquent when they’re not paid up in full,” the US president said.

Only nine of Nato’s 29 members spend 2% of their GDP on defence. Trump cited Germany as falling short, spending only 1% to 1.3%.

Leaders of the 29 allies are meeting in London to discuss the overall spending levels and nature of the threat to Nato, including the degree to which Russia, other countries such as China, or non-state actors represent the primary threat.

It is the first time that Nato has formally discussed the rise of China, Stoltenberg said, and is a marked departure for an organisation focused on European and North Atlantic security. But the Nato boss sought to downplay its significance: “We are not going to establish a new adversary”.

Trump indicated his willingness to work with the Russian president, Vladimir Putin, a view he shares with Macron. He said: “I have to say this, Russia wants to make a deal on arms control ... Russia wants to make a deal as recently as, like, two weeks ago. Russia wants very much to make a deal on arms control and nuclear.”

Trump also said Iran was killing thousands of people for protesting and urged the world to take greater notice.

Amnesty International has claimed that more than 200 have been killed in the disturbances caused by a surprise doubling of petrol prices – but Tehran describes the figures as lies.

Trump said: “Iran is killing, perhaps, thousands and thousands of people right now as we speak, that is why they cut off the internet, so people can’t see what is going on.

“Not just small numbers, which are bad – big numbers, which are really bad, and really big numbers ... It is a terrible thing and the world has to be watching.”

Trump also said a trade agreement with China might have to wait until after the US presidential election in November 2020, denting hopes of a quick resolution to the dispute that has weighed on the world economy.

“I have no deadline, no. In some ways, I think it’s better to wait until after the election with China,” Trump said. “But they want to make a deal now, and we’ll see whether or not the deal’s going to be right – it’s got to be right.”