The U.S. Justice Department issued a warrant to seize an Iranian oil tanker detained in Gibraltar, a day after a judge in the British overseas territory ordered its release.

The U.S. move late Friday deepens a weeks-long diplomatic dispute between Tehran and Washington. It also comes amid a standoff between the two countries after President Donald Trump withdrew from an international nuclear accord with Tehran and reimposed sanctions. Tensions in the Persian Gulf have been on the rise since.  

The tanker “Grace 1” was seized last month in a British Royal Navy operation off the coast of Gibraltar. Authorities suspected it of violating European Union sanctions on oil shipments to Syria. Its seizure aggravated fears of a conflict in the Persian Gulf, where Iran claims control of the Strait of Hormuz, a strategic waterway for oil shipments. 

But despite a last-minute U.S. attempt on Thursday to keep the oil tanker detained in Gibraltar, a court there ordered its release. Gibraltar’s Chief Minister Fabian Picardo said he had received “written assurances” from Iran that its tanker would not seek to travel to a destination that is subject to EU sanctions. “This assurance has the effect of ensuring that we have deprived the (President Bashar) Assad regime in Syria of more than one hundred and forty million dollars of valuable crude oil,” he said. 

The “Grace 1” was carrying 2.1 million barrels of Iranian crude oil. 

It wasn’t immediately clear if the tanker has tried to depart Gibraltar. 

The warrant unsealed in the U.S. District Court in Washington alleges “all petroleum aboard it and $995,000.00 are subject to forfeiture based on violations of the International Emergency Economic Powers Act (IEEPA), bank fraud statute, and money laundering statute, as well as separately the terrorism forfeiture statute.”

Federal prosecutor Jessie Lui claimed Iran had used a “network of front companies” to launder money used to ship its oil abroad in violation of sanctions and that these companies had links to Iran’s Revolutionary Guard Corps, an organization with ties to Iran’s military and economy that the U.S. has placed on a terrorism list.

There was no immediate response from Iran or the United Kingdom.

Tehran has called the impounding of “Grace 1” an “illegal interception.”

Iran tensions: Gibraltar defies U.S. intervention and releases Iran oil tanker

Still, the attempted U.S. intervention Friday may further strain the situation in the Persian Gulf. After Gibraltar’s detention of “Grace 1,” Iran seized the British-flagged oil tanker “Stena Impero” as it passed through the Strait of Hormuz. The Islamic Republic is still holding the “Stena Impero,” claiming it failed to stop after colliding with an Iranian fishing boat. Iran has also seized other foreign oil tankers and the U.S. blames Tehran for the downing of a U.S. surveillance drone as well as a series of sabotage attacks on ships operating in the Persian Gulf. Iran disputes claims it was involved. 

The Trump administration late last month sanctioned Iran’s foreign minister, Mohammad Javad Zarif, in a move that has narrowed the window for dialogue with Iran. It was a decision that came after weeks of heated rhetoric between the U.S. and Iran as Washington has attempted to squeeze the regime economically and isolate it diplomatically. European signatories to the nuclear deal brokered during President Barack Obama’s administration, including Britain, have so far resisted pressure from Washington to abandon the landmark 2015 agreement that placed restrictions on Iran’s nuclear program in return for easing sanctions on its key industries, such as oil. 

“A successful U.S. seizure of the Grace 1 tanker in Gibraltar would further increase tensions between Iran and the Trump administration,” said Rocky Weitz, the director of Maritime Studies at The Fletcher School of Tufts University.

Authorities in Gibraltar said they decided to release “Grace 1” to ease tensions. Legal action against the ship’s crew and captain, an Indian national, were dropped.

Sanctions: Trump administration targets Iran’s foreign minister in new escalation

New British Prime Minister Boris Johnson, who enjoys a good relationship with Trump, has not indicated publicly whether he intends to back Washington’s “maximum pressure” strategy over Iran that has stoked fears it could lead to military conflict. The two leaders will hold their first meeting since Johnson’s elevation to prime minister on the sidelines of a Group of Seven summit in Biarritz, France, on Aug. 24-26. 

The U.S. State Department has meanwhile threatened a visa ban or potential prevention of admission to the U.S. on anyone who assists the “Grace 1.”