HELSINKI — The U.K. is ready to work with its European allies to intercept vessels in Russia’s “shadow fleet,” Britain’s chief foreign minister said Wednesday.

A week after British armed forces supported the U.S. seizure of a Russian-flagged oil tanker in the North Atlantic, Yvette Cooper said Britain is prepared to work on enforcement with “other countries and other allies” against ships suspected of carrying sanctioned oil or damaging undersea infrastructure.

Promising “stronger action” to break the shadow fleet’s “chokehold,” she added: “It means a more robust response, and it means as we see operations by shadow fleet vessels, standing ready to be able to act.”

While the foreign secretary would not be drawn on the specific action the U.K. might take, her charged rhetoric appears to be laying the ground for future interventions that go beyond last week’s coordination with the Trump administration.

Officials believe that the U.K. government has identified a legal basis for the military to board shadow fleet vessels in international shipping lanes, in certain cases.

Cooper did not rule out the prospect of British forces boarding vessels, telling POLITICO: “It means looking at whatever is appropriate, depending on the circumstances that we face.”

She also did not rule out using oil from seized vessels to fund the Ukrainian war effort — but cautioned that the prospect was of a different order to using frozen Russian assets to fund Ukraine. That idea hit a wall in discussions between EU countries in December.

The foreign secretary said: “As you know, we’ve had all sorts of discussions in the past about different Russian sovereign assets. That’s a different set of circumstances. So we take the approach that it always has to be done within an international legal framework and on a case-by-case basis.”

Asked directly if she was talking about joint shadow fleet operations with European allies, Cooper said: “We stand ready to work with allies on stronger enforcement around the shadow fleet.”

Cooper made her comments on Wednesday after a demonstration on board the Finnish Border Guard ship Turva. It took part in a Dec. 31 operation to seize a cargo ship sailing from Russia to Israel, which was accused of deliberately damaging a cable between Helsinki and Estonia.

Finnish authorities demonstrated a mock operation similar to the one that seized the ship on New Year’s Eve. Cooper watched as five armed officers slid down a rope from a helicopter onto the deck and stormed the bridge, shouting: “Hands up.” The operation took around three minutes.

Cooper said after the demonstration: “The reason for being here is to see the work that Finland has been doing around the shadow fleet, and to look at what the further potential is for us to work with allies to strengthen that enforcement work.”

She name-checked work by France and Finland, while one U.K. official said she also intends to work with Norway.

Mari Rantanen, Finland’s interior minister, said the age of some Russian-linked tankers using northern shipping routes risks an ecologically disastrous oil spill. “These vessels, these tankers, are very old,” she told POLITICO. “They are not built [for] this kind of icy weather, and they are in very bad shape, so the environmental risk is huge.”

Mikko Simola, the commander of the Gulf of Finland coastguard, said he has seen “a rapid change since early 2022” in the prevalence of malign activity, for which Moscow denies responsibility.

Simola said he would let the courts decide who was culpable, but said it was “certainly very strange to believe that in a short period of time, many cable and gas pipe damages would happen by accident in the same area.”