The United States has seized a Russian-flagged oil tanker in the North Atlantic after a weeks-long pursuit, in a high-profile enforcement action that has drawn sharp criticism from Moscow and highlighted close military cooperation between Washington and London.

US officials said the tanker, suspected of being part of a network used to evade Western sanctions, was intercepted in international waters and taken under American control without incident. The vessel had previously been linked to the transport of oil associated with sanctioned regimes, according to US authorities.

The operation involved US naval and law-enforcement assets tracking the ship across thousands of miles of ocean before boarding it earlier this week. Officials described the seizure as lawful and consistent with international rules aimed at preventing sanctions evasion and illicit shipping practices.

The UK government confirmed on Wednesday that it had provided support to the mission. A spokesperson for the Ministry of Defence said British forces assisted with surveillance and coordination during the operation but did not take part in boarding the tanker.

“The UK works closely with our allies to uphold international law and counter attempts to undermine global security,” the MoD said, adding that the action demonstrated the strength of trans-Atlantic defence cooperation.

Russia reacted angrily, condemning the seizure as a violation of maritime law and accusing the US of acting unlawfully against a vessel flying the Russian flag. Russian officials warned the move risked escalating tensions at sea and said they were seeking further details through diplomatic channels.

The seizure comes amid intensified efforts by the US and its allies to crack down on so-called “shadow fleets” — tankers that operate under opaque ownership structures, frequently change names or flags, and are used to move oil in defiance of international sanctions.

Analysts say the action signals a more assertive Western stance on maritime sanctions enforcement, at a time of already strained relations between Russia and the West over the war in Ukraine and broader geopolitical disputes.

US officials have not yet said where the tanker will be taken or what legal steps will follow, but indicated that investigations into its ownership, cargo and trading history are continuing.