US authorities have seized Iranian oil from four tankers that were en route to Venezuela, the first action of its kind, and the latest escalation of American-led sanctions pressure on maritime trade.

The Department of Justice announced on August 14 it had confiscated more than 1.1 million barrels of fuel it says was being shipped to Venezuela by the Islamic Revolutionary Guard Corps (IRGC) – an elite military unit in Iran that is designated as a terrorist organisation by the US government.

The raid is the latest in a series of US sanctions efforts linked to maritime trade – a trend that has accelerated since the publication of a landmark advisory in May by the country’s sanctions regulator – and the government says that policy is starting to bear fruit.

“We are seeing more and more global shipping fleets avoiding the Iran-Venezuela trade due to our sanctions implementation and enforcement efforts,” says Morgan Ortagus, a spokesperson for the US State Department.

“The United States remains committed to our maximum pressure campaigns against the Iranian and Maduro regimes.”

Though Venezuela sits on vast oil reserves, it no longer has any functioning refineries and so has become reliant on importing petroleum. In Iran, whose oil exports are subject to the US’ fearsome sanctions regime, it has a willing seller.

The US has previously attempted to disrupt transactions between the two sanctioned countries, but with little success. In May, it emerged that five tankers had successfully made their way from Iran to Venezuela, despite threats of intervention from the US.

American officials quickly retaliated with fresh sanctions against the five captains of those ships, with Secretary of State Mike Pompeo warning: “Their careers and prospects will suffer from this designation.”

However, this month’s action is the first use of the US’ civil forfeiture process in combination with sanctions against a designated terrorist organisation as the legal basis for seizing cargo.

Evelyn Sheehan, a lawyer at Kobre & Kim, says that legal mechanism allows authorities to confiscate assets – in this case oil aboard the four vessels – without pursuing the individual criminal actors themselves.

“What’s striking is that with most forfeiture actions, you would have to prove the link between those assets and the criminal act,” she tells GTR.

“This case is different because it’s terrorism-related. The US government has the ability to go after any assets owned by a designated terrorist organisation, anywhere in the world, without having to prove that those assets are being used to carry out a criminal act. It’s a particularly heavy sledgehammer the US can wield.”

Beau Barnes, a lawyer at the same firm, says he sees the action as “a tactical legal innovation”.

“It’s part of this all-tools approach, using a combination of criminal tools, civil tools, diplomatic tool and sanctions,” he says.

“There’s also a political-diplomatic element here. By its designation of the IRGC and Quds force, the US government essentially takes the position that the government of Iran is a terrorist organisation.

“That opinion is certainly not shared around the world, or even among US allies, so the use of the terrorism authority is a way to put the hammer down a little bit more against Iran. Calling this a terrorism-related enforcement action gives it a little bit more oomph.”

Though the US government was issued with a warrant for the forfeiture in early July, it must now prove the allegations of ties to terrorist activity through the courts. The case is being investigated by Homeland Security Investigations and the FBI, and handled by the National Security Division and the US Attorney’s office for the District of Columbia.

The four vessels that were raided are all Liberia-flagged tankers. The location of three of the ships – the Bella, Bering and Luna – has not been detected since May according to MarineTraffic data. In each case, the last recorded port call was in the United Arab Emirates.

The fourth ship, the Pandi, has not transmitted its location since late June and was last detected in waters near Oman.

Hojat Soltani, Iran’s ambassador to Venezuela, tweeted after the raid that the ships are not registered to or owned by Iranian entities, and described the action as “another lie and psychological warfare from the propaganda machine of the US”.

“The terrorist Trump cannot make up for his humiliation and defeat against Iran with false propaganda,” Soltani wrote.

The US said an “unrelated” fifth vessel, the Wila, was forcibly boarded by the Iranian navy “in an apparent attempt to recover the seized petroleum, but was unsuccessful”.

Grainy video footage released by US Central Command – part of the Department of Defense –appears to show figures abseiling from a helicopter onto the deck of the ship.

As of press time, the Wila’s location had not been reported for over 24 hours but was last recorded in the Gulf of Oman.