Iranian spies were foiled as they plotted to carry out an assassination in northern Europe, according to officials in Denmark.

“The assessment is that an Iranian intelligence agency has planned an assassination on Danish soil,” Danish Foreign Minister Anders Samuelsen said Tuesday. “This is completely unacceptable. In fact, the gravity of the matter is difficult to describe.”

Iranian officials targeted the leader of the Arab Struggle Movement for the Liberation of Ahwaz, an organization that seeks to carve an Arab state out of western Iran. The Danish Security and Intelligence Service detected someone “conducting reconnaissance” of the target’s house last month, leading to an investigation and an arrest.

“With regard to the man in custody, an investigation is ongoing,” the Danish security service announced. “However, when PET now states that there is sufficient basis to conclude that an Iranian intelligence service has been planning the assassination of an individual living in Denmark, it is not only based on the case against the man in custody, but on comprehensive intelligence efforts.”

Denmark’s announcement marks at least the second time this year that Iran has been accused of plotting an attack on European soil. An Iranian diplomat credentialed in Austria was allegedly part of a plot to bomb a Paris rally of Iranian dissidents in June.

“This happened just as the regime has been putting a full-court press on European countries to stay in the nuclear deal,” Secretary of State Mike Pompeo said last month in New York. “As a just response to this support for terrorism, a few weeks ago our ally France indefinitely postponed all non-essential diplomatic travel to Iran. That is a good first step, and we hope to see more actions like this from other nations. We must put pressure on the regime to rein in its destruction and demand that Iran act like a normal country.”

President Trump’s renewed sanctions on Iran’s oil industry are scheduled to take effect next week. Iranian officials denied the Danish allegations, maintaining that they are another effort to undermine Western support for the 2015 nuclear deal.

“Iranian Foreign Ministry Spokesman Bahram Qassemi said on Tuesday that the publication of such ‘spiteful’ media reports and its attribution to Iran is a plot by enemies to affect Tehran’s growing relations with European countries,” Iran’s state-run media said. “He emphasized that these claims are the continuation of plots and conspiracies hatched by known enemies who are against good and expanding Iran-Europe relations at the current sensitive conditions.”

European leaders — particularly France, Germany, and the United Kingdom — are lobbying for Trump to avoid the most aggressive implementation of the coming sanctions.

“The main risk of the pressure strategy is that it pushes Iran to restart its nuclear activities,” French President Emmanuel Macron said last month, according to Reuters. “So if we want a chance for this pressure on Iran to produce results, then we need to make sure it doesn’t opt for the worst option.”