Argentina’s President, Javier Milei, has heightened tensions with the UK over the Falklands, warning he will impose economic sanctions on firms which exploit oil close to the British overseas territory.

In a national address, Milei said the “winds of change” favoured Argentina’s claim to the islands, emboldened by signs the US might reconsider its stance of neutrality on the issue.

Milei said the Sea Lion oilfield, where a British and an Israeli firm are due to begin exploration work, was a “clear and present danger”.

More than 40 years after the war over the territory, sovereignty of the archipelago in the south-west Atlantic Ocean remains hotly disputed between Britain and Argentina.

BBC News is seeking comment from the UK government on Milei’s statement. It reiterated this week that Falkland islanders “are British with a right to determine their own future”.

Milei announced his sanctions plan surrounded by his cabinet at the presidential palace, reiterating that the Falklands were “historically and legally” Argentinean.

The crux was a proposal to introduce a bill to parliament to impose sanctions on energy firms planning to begin oil exploration off the Falklands.

The Sea Lion oilfield, around 130 miles from the islands, contains an estimated 1.7 billion barrels of oil.

Two oil companies – Britain’s Rockhopper Exploration and Israel’s Navitas Petroleum – are due to begin exploration work in the Sea Lion oilfield off the Falklands coast within two years, something Milei said Argentina could not allow.

“If we do not act swiftly in a few months, they will have the material capacity to access the oil that lies underneath our sea – something that has never happened before,” Milei said.

It is not yet clear how or to what extent the sanctions will be imposed or tightened. In Argentina, a current law already allows for restrictions of activities of oil companies participating in projects considered unauthorised on the islands.

In a 2013 referendum, residents of the islands voted overwhelmingly to remain a British overseas territory – with 99.8% voting in favour and just three voting against.

When asked in an interview with GB News earlier on Thursday if the US would “come to the UK’s aid” during a potential territorial dispute, Trump replied that Britain had “not been there to help me” with his country’s war with Iran.

Trump’s latest remarks come after he previously said the US could revise its neutral position on the islands’ sovereignty if the UK failed to increase defence spending.

Milei also announced plans for a naval base in Tierra del Fuego in the south of Argentina, saying Trump and the US were “assessing their historical position in regard to the Falklands”, which Argentina refers to as Las Malvinas.

“That is not empty words as they understand that Argentina is a reliable partner and a valuable ally for decades to come,” he said, describing the UK as a nation “in decline”.

“There are winds of change in the world that are favourable to our claim,” Milei added.

Hours before Milei’s speech, Sarah Rogers, a senior official in the US state department, posted a photo of herself with Milei alongside the caption: “Exciting things are happening in the Western Hemisphere.”

Argentine forces landed on the Falklands to stake a territorial claim in 1982, sparking a 74-day war over the territory as a British military task force ejected them.

The conflict led to the deaths of 255 British military personnel, three islanders and 649 Argentine military personnel.

In a sign of the ongoing tensions between the two countries over the issue, Argentina players celebrated their victory over England in the World Cup by waving a banner in support of their country’s territorial claim.