Britain will sanction up to 100 oil tankers from Russia's so-called shadow fleet, seeking to step up pressure on Moscow in the war with Ukraine as leaders from the Joint Expeditionary Force (JEF) security alliance meet in Norway on Friday.
The vessels carried cargo worth over $24 billion since the beginning of last year, with some ships also involved in damaging critical infrastructure, Britain said.
The sanctions would represent Britain's largest package of measures against the shadow fleet, Prime Minister Keir Starmer's office said. It did not provide further detail but similar sanctions previously have restricted or prohibited the movement of shadow fleet vessels and their access to some British ports.
Starmer will announce the measures at a summit of JEF leaders in Oslo. Founded in 2014, the British-led group of 10 northern European nations including Denmark, the Netherlands, Finland and Sweden, acts as a supplement to wider cooperation through the NATO military alliance.
"Every step we take to increase pressure on Russia and achieve a just and sustainable peace in Ukraine is another step towards security and prosperity in the UK," Starmer said.
"We will do everything in our power to destroy (President Vladimir Putin's) shadow fleet operation, starve his war machine of oil revenues and protect the subsea infrastructure that we rely on for our everyday lives."
The vessels are suspected of damaging undersea infrastructure in Europe, including power lines, cables and pipelines in the Baltic Sea. Nordic and Baltic nations, alarmed by rising sabotage risks, have pressed for action.
JEF nations have been steadfast supporters of Ukraine, with many spooked by Russia's invasion of its smaller neighbour. Sweden and Finland have also joined NATO, upending decades of non-alignment.
Summit host Norway is a top exporter of gas to Britain and continental Europe, delivered via its 8,800-kilometre (5,468-mile) North Sea pipeline network, which is surveilled by NATO navy patrols to protect against sabotage.
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