US President Donald Trump said he is considering applying new tariffs on countries that oppose his ambition of annexing Greenland, CNN reported. 

Trump has insisted for months that the US should control the world’s largest island, declaring earlier this week that anything less would be “unacceptable.”

He argued that the US acquiring the Arctic landmass, a semiautonomous territory of NATO ally Denmark, would serve its national security interests and in turn strengthen the defense alliance.

Speaking Friday at the White House, Trump suggested punishing countries with tariffs if they don’t back that plan.

“I may put a tariff on countries if they don’t go along with Greenland, because we need Greenland, because we need Greenland for national security,” Trump said during an event focused on health care. The president made the remark as he recounted using tariffs to force other nations to cooperate on a plan to lower drug prices in the United States.

This was the first time he proposed using import taxes to advance the Greenland issue, but he did not specify which countries might be targeted or what authority he would invoke.

Greenland is rich in natural resources, including oil, gas and rare earth minerals. Its location between North America and the Arctic makes it ideal for missile early-warning systems and monitoring vessels in the region.

US control of the territory faces opposition not only from Denmark and Greenland but also from several other nations – and is supported by only 25% of Americans, according to a CNN poll.

By raising the prospect of NATO’s largest and most powerful member buying or annexing the territory of another, Trump’s declarations have thrown Europe’s decades-old, US-led security alliance into crisis.

His push to control Greenland has prompted outrage among European nations.

Denmark warned an attack on Greenland would effectively end NATO, and announced on Wednesday that it was expanding its military presence there, “in close cooperation with NATO allies.”

France, Germany, the Netherlands, Finland, Norway and Sweden have all since confirmed the deployment of military personnel to the island. 

Canada and France have said they plan to open consulates in Nuuk, Greenland’s capital, in the coming weeks.