Denmark’s prime minister has told Donald Trump to “stop the threats” about taking over Greenland.

Mette Frederiksen said “it makes absolutely no sense to talk about the need for the United States to take over Greenland”, adding: “The US has no right to annex any of the three nations in the Danish kingdom.”

Her remarks come after Katie Miller – the wife of one of Trump’s aides, Stephen Miller – tweeted a map of Greenland in the colours of the American flag alongside the word “SOON”.

Trump has repeatedly raised the possibility of Greenland becoming an annexed part of the US, citing its strategic location and mineral wealth, and doubled down on his claims after Frederiksen’s comments.

In her statement, posted on the Danish government website, Frederiksen said she was addressing the US “very directly”.

She said Denmark – “and thus Greenland” – was a Nato member and covered by the alliance’s security guarantee. Denmark already had a defence agreement with the US that gave it access to Greenland, she said, and Denmark had upped its investment in security in the Arctic region.

“I would therefore strongly urge the United States to stop the threats against a historically close ally and against another country and another people who have very clearly said that they are not for sale,” she said.

Hours later on Air Force One, Trump reiterated the goal. “We need Greenland from the standpoint of national security, and Denmark is not going to be able to do it,” he said.

Earlier the Danish ambassador to the US responded to the post by Ms Miller – a right-wing podcaster and former aide to Trump during his first term – with a “friendly reminder” that the two countries were allies and saying Denmark expected respect for its territorial integrity.

The exchange comes after the US carried out a major military operation against Venezuela on Saturday, capturing its president Nicolas Maduro and his wife and removing them to New York.

Trump later said the US would “run” Venezuela and US oil companies would “start making money for the country”.

The US president has previously refused to rule out using force to secure control of Greenland, a vast island in the Arctic.

Trump has claimed that making it part of the United States would serve US security interests due to its strategic location and its abundance of minerals critical to high-tech sectors.

The Trump administration’s recent move to appoint a special envoy to Greenland prompted anger in Denmark.

Greenland, which has a population of 57,000 people, has had extensive self-government since 1979, though defence and foreign policy remain in Danish hands.

While most Greenlanders favour eventual independence from Denmark, opinion polls show overwhelming opposition to becoming part of the US.

University professor Xavier Crettiez admits that he doesn’t know the real names of many of the students on his course.

This is a highly unusual state of affairs in the world of academia, but Prof Crettiez’s work is far from standard.

Instead, he helps train France’s spies.

“I rarely know the intelligence agents’ backgrounds when they are sent on the course, and I doubt the names I’m given are genuine anyway,” he says.

If you wanted to create a setting for a spy school, then the campus of Sciences Po Saint-Germain on the outskirts of Paris seems a good fit.

With dour, even gloomy-looking, early 20th Century buildings surrounded by busy, drab roads and large, intimidating metal gates, it has a very discreet feel.

Where it does stand out is its unique diploma that brings together more typical students in their early 20s, and active members of the French secret services, usually between the ages of 35 and 50.

The course is called Diplôme sur le Renseignement et les Menaces Globales, which translates as Diploma of Intelligence and Global Threats.

It was developed by the university in association with the Academie du Renseignement, the training arm of the French secret services.

This came following a request from French authorities a decade ago. After the 2015 terrorist attacks in Paris, the government went on a large recruitment drive within the French intelligence agencies.

It asked Sciences Po, one of France’s leading universities, to come up with a new course to both train potential new spies, and provide continuous training for current agents.

Large French companies were also quick to show an interest, both in getting their security staff onto the course, and snapping up many of the younger graduates.

The diploma is made up of 120 hours of classwork with modules spread over four months. For external students – the spies and those on placement from businesses – it costs around €5,000 ($5,900; £4,400).

The core aim of the course is to identify threats wherever they are, and how to track and overcome them. The key topics include the economics of organized crime, Islamic jihadism, business intelligence gathering and political violence.

To attend one of the classes and speak to the students I had to be vetted first by the French security services. The theme of the lesson I joined was “intelligence and over-reliance on technology”.

One of the students I speak to is a man in his 40s who goes by the name Roger. He tells me in very precise, clipped English that he is investment banker. He adds: “I provide consultancy across west Africa, and I joined the course to provide risk assessments to my clients there.”

Prof Crettiez, who teaches political radicalisation, says there has been a huge expansion in the French secret services in recent years. And that there are now around 20,000 agents in what he called the “inner circle”.

This is made up of the DGSE, which looks at matters overseas, and is the French equivalent of the UK’s MI6 or the US’s CIA. And the DGSI, which focuses on threats within France, like the UK’s MI5 or the US’s FBI.

But he says it’s not just about terrorism. “There are the two main security agencies, but also Tracfin an intelligence agency which specializes in money laundering.

“It is preoccupied with the surge in mafia activity, especially in southern France, including corruption in the public and private sectors mainly due to massive profits in illegal drug trafficking.”

Other lecturers on the course include a DGSE official once located in Moscow, a former French ambassador to Libya, and a senior official from Tracfin. The head of security at the French energy giant EDF also runs one module.

The private sector’s interest in the diploma is said to be continuing to grow. Big businesses, especially in the defence and aerospace sector, but also French luxury goods firms, are increasingly keen to hire the students as they face relentless cybersecurity and spying threats as well as sabotage.

Recently graduates have been snapped up by the French mobile phone operator Orange, aerospace and defence giant Thales, and LVHM, which owns everything from Louis Vuitton and Dior to champagne brands Dom Perignon and Krug.

Twenty eight students are enrolled in this year’s class. Six are spies. You can tell who they are, as they are the ones huddled together during class breaks, away from the young students, and not too overwhelmed with joy when I approach them.

Without saying their exact roles, and with arms crossed, one says the course is considered a fast-track stepping stone for a promotion from the office to field work. Another says he gets fresh ideas being in this academic environment. They signed the day’s attendance form with just their first names.

One of the younger students, Alexandre Hubert, 21, says he wanted a deeper understanding of the looming economic war between Europe and China. “Looking at intelligence gathering from a James Bond viewpoint is not relevant, the job is analysing risk and working out how to counteract it,” he tells me.

Another class member, Valentine Guillot, also 21, says she was inspired by the popular, fictional French TV spy drama Le Bureau. “Coming here to discover this world which I didn’t know anything about except for the TV series has been a remarkable opportunity, and now I am very keen to join the security services.”

Nearly half of the students in the class are in fact women. And this is a relatively recent development according to one of the lecturers, Sebastien-Yves Laurent, a specialist on technology in spying.

“Women’s interest in intelligence gathering is new,” he says. “They are interested because they think it will provide for a better world.

“And if there is one common thread amongst all these young students it’s that they are very patriotic and that is new compared to 20 years ago.

If you are keen to apply to get on the course, French citizenship is an essential requirement, although some dual citizens are accepted.

Yet Prof Crettiez says he has to be wary. “I regularly get applications from very attractive Israeli and Russian women with amazing CVs. Unsurprisingly they are binned immediately.”

In a recent group photo of the class you can immediately tell who the spies are – they had their backs to the camera.

While all the students and professional spies I met are trim and athletic, Prof Crettiez is also keen to dispel the myth of James Bond-like adventure.

“Few new recruits will end up in the field,” he says. “Most French intelligence agencies jobs are desk bound.”