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Asked which world leader – Angela Merkel or Vladimir Putin – better represented their country’s interests, Donald Trump once again displayed an impulse to lash out at an ally while holding back criticism of an adversary.

The comments are the latest in a string of attacks on the German chancellor, who Trump slammed at the outset of the Nato summit in Brussels last week.

“Angela was a superstar until she allowed millions of people to come into Germany,” Trump said in an interview with the Fox News host and Trump ally Tucker Carlson, which aired on Tuesday. “That really hurt her badly, as you know. She was unbeatable in any election. She allowed millions of people to come in … it was the great migration and obviously, it’s hurt Angela very much.”

In conclusion, Trump said: “I don’t want to say who is better and who is not, but I will say this, she’s been very badly hurt by immigration – very, very badly.”

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The interview was taped in Helsinki on Monday, after Trump’s stunning remarks at a press conference with Putin, during which he seemingly accepted the Russian leader’s denial of election meddling. On Tuesday, Trump sought to clarify his comments by stating that he accepted the assessment of US intelligence agencies that Russia interfered in the 2016 US election – while moments later questioning who was responsible.

In the wide-ranging Fox interview, Trump questioned the merit of the Nato military alliance, which was formed as a guard against Soviet aggression, and the principle of collective defense.

“Why should my son go to Montenegro to defend it from attack?” Carlson asked.

Trump replied: “I understand what you’re saying. I’ve asked the same question … Montenegro is a tiny country with very strong people,” he said. “They may get aggressive and congratulations, you’re in world war III.”

Trump’s comments echo remarks he made in Brussels, which rattled members and raised questions about the durability of an alliance that has been a defining feature of American foreign policy since the second world war.

In the interview, Trump also took credit for pushing Nato members to increase spending on their defense budget but once again appeared confused about how the alliance works.

“The secretary-general said that because of President Trump, last year we had an additional $44bn – with a B – billion dollars raised for Nato and this year, it’s going to be much more than that, and the countries all agreed,” Trump bragged.

But the countries aren’t raising money for Nato, as Trump suggested. Rather, Nato members are inching closer to a commitment made in 2014 to spend 2% of each country’s gross domestic product on its defense by 2024. During the Brussels summit, the members again pledged to meet that goal.

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Trump, who has embraced hardline immigration policies, called Europe’s immigration policies a “disaster”.

During his time in Brussels and the UK, Trump said, he had told European leaders that their immigration policies were “destroying the culture of Europe”.

“The crime is up in those areas and you better do something. I told them that,” he said. “I said: ‘Look, it’s not me. It’s not anything. You just look at the numbers. The numbers speak. But the culture is changing rapidly and the crime rate is changing more than rapidly, you better do something.’ I told them that.”

When asked to name one place on the continent that had been improved by immigration, Trump said he could not.

Trump reiterated his call for a border wall between the US and Mexico and incorrectly accused Democrats of being for “open borders”. Asked why Democrats favored a more welcoming immigration policy, Trump replied: “Maybe it’s political philosophy that they grew up with, maybe they learned it at school, maybe they’re fools, I don’t know.”

He also lashed out at the former CIA director John Brennan, who called the president’s behavior in Helsinki “treasonous”.

“I think Brennan is a very bad guy and if you look at it a lot of things happened under his watch,” Trump said. “I think he’s a very bad person.”