Argentine President Javier Milei has threatened legal action against the U.K. over an oil exploration project off the coast of the Falkland Islands, a British overseas territory at the center of a yearslong sovereignty dispute.

Setting a two-week deadline on Monday, Milei said if the Sea Lion offshore oil project—a venture between British and Israeli companies—has not been stopped, then he will take the matter to the “International Tribunal for the Law of the Sea,” an independent judicial body responsible for the application of the United Nations' Convention on the Law of the Sea. Milei said Argentina’s Foreign Ministry and legal teams have been prepped to “initiate international arbitration” against the U.K. The oil companies—Britain's Rockhopper Exploration and Israel's Navitas Petroleum—have said they have valid licenses for the planned extractions. TIME has reached out to the companies for further comment.

The threat from Milei comes after weeks of escalating tensions between Argentina and the U.K. The Falkland Islands, known in Argentina as “Islas Malvinas,” sit roughly 300 miles from the Argentine coast, and have been a British territory for almost two centuries, but Argentina has never accepted British sovereignty over the area. As such, Milei is adamant the oil exploration project would amount to an “illegal plundering of [Argentina’s] resources.”U.S. President Donald Trump further stoked tensions in late August when he suggested the U.S. is reviewing its neutrality on the sovereignty dispute.

In response to Milei’s deadline, a spokesperson for the U.K.’s Foreign Office told TIME: "These latest threats are another attempt to deny the Falkland Islanders the right to pursue their own economic development and decide their own future. As the Prime Minister [Andy Burnham] has made clear, our support for the Falkland Islanders' right of self-determination is unwavering, and we will always defend it.”In a 2013 referendum, Falkland residents voted overwhelmingly in favor of remaining under British rule.

The referendum saw 92% of all eligible voters turn out, with 99.8% of votes being “yes.”Yet the topic continues to be a rallying cry in Argentina for Milei, as it was for the Argentine Presidents that came before him.

There are vast energy reserves in the Sea Lion oilfield, with the basin estimated to contain 1.7 billion barrels of oil.

“The Argentine Republic will not stand idly by and will continue to firmly defend its sovereign rights, using all the diplomatic, administrative and legal tools recognized by national and international law,” read a presidential statement condemning the oil exploration project. The statement said that Milei’s office would seek to initiate arbitral proceedings under the United Nations Convention on the Law of the Sea (UNCLOS)—an international treaty that sets rules for the use and governance of the world's oceans and their resources.

Marcelo Kohen, a professor emeritus of international law at the Geneva Graduate Institute of International and Development Studies, says the two-week ultimatum is tied to a procedure Argentina has to follow before seeking provisional measures from the tribunal.

“Argentina gave the U.K. two weeks to accept the provisional measures requested by Argentina, and in case the U.K. doesn't comply with this request, Argentina will go to the International Tribunal for the Law of the Sea in Hamburg,” he says. Once the case has been taken to the judicial body, “both parties will go to the tribunal, and there will be oral hearings, and then the tribunal will decide whether the United Kingdom must stop the exploitation of hydrocarbon resources.”This isn’t the first time Argentina has turned to international legal mechanisms to resolve maritime disputes. In October 2012, Argentina initiated arbitration after Ghana detained an Argentine naval training ship. The tribunal unanimously ordered Ghana to release the vessel.

However, experts tell TIME that the international court has previously said it does not view it to be within its jurisdiction to rule on territorial sovereignty disputes.

Milei’s turn to international law comes amid his fierce criticism of the United Nations, which he has accused of looking "the other way" amid escalating tensions with Britain.

During his speech at the General Assembly in New York last week, Milei called the U.N. a “useless organization, whose only purpose is to sustain a cast of fatally arrogant parasites masquerading as well-intentioned bureaucrats.”“The Sea Lion project is moving towards oil extraction on the Argentine continental shelf in the North Malvinas basin under British licenses that Argentina rejects as illegal and illegitimate,” he told delegates.

The ultimatum is the latest escalation from Milei over the Falklands. Earlier in September, he announced that Argentina would sanction oil companies operating in the area and strengthen the government’s ability to respond to what it views as threats against its sovereignty.

Following through on that threat, Argentina's Foreign Ministry on Sept. 15 said it had sent approximately 180 warning and risk assessment notes to individuals and legal entities located in 30 countries and filed three criminal complaints involving 10 companies and their directors. TIME has reached out to Argentina's Foreign Ministry for further information. Do the Falklands belong to Britain or Argentina?

The Falklands have been administered by Britain since 1833 and remain a British overseas territory, but Argentina continues to claim them. The Falkland Islands have been on the U.N.’s list of non-self-governing territories since 1946.

In 1982, Argentina invaded the Falklands, sparking a bloody 74-day conflict that Britain ultimately won.

Matthew Benwell, a reader in human geography at Newcastle University, tells TIME that the Falklands remain a “largely self-governing British overseas territory.”“Aside from the U.K.'s provision of defense and foreign affairs—the islanders continue to rely on that assistance—the U.K. government and political parties across the spectrum support the islanders' right to self determination,” he says, referencing the 2013 referendum. But Argentina approaches the sovereignty question through a different legal framework.

Kohen says that Buenos Aires regards it as “a case of de-colonization” in which self-determination does not apply. Argentina points to a 1965 U.N. Resolution, which calls for bilateral sovereignty negotiations and consideration of the islanders’ “interests,” but does not expressly apply the principle of self-determination to them.

The Argentine permanent mission to the U.N. similarly characterizes the issue as a “colonial situation” and “reiterates its willingness to resume bilateral negotiations with the United Kingdom in order to reach a peaceful resolution to this sovereignty dispute.”

Benwell, however, describes the sovereignty argument as a “persistent one-way debate between the U.K. and Argentina, that predominantly comes from Argentina.”

How Trump stoked Falklands tensions Trump, a long-time ally of Milei’s, prompted discussion in late August when he told reporters in the Oval Office that the U.S. was reviewing its position regarding the U.K.’s administration of the islands. “I always review every position. That's just one of many,” Trump said.

Milei appeared to seize on Trump's comments during a national address on Sept. 3, insisting that the “winds of change blowing around the world” are favorable to Argentina’s long-standing sovereignty claim over the South Atlantic archipelago.

In an interview with a British broadcaster, Trump declined to offer a clear pledge of support while emphasizing the islands’ distance and tying the issue to Britain’s refusal to aid the U.S. in the Iran war. “Your country was not there to help me,” he insisted.

On Sept. 12, Trump, during a sit-down with Irish Taoiseach (Prime Minister) Micheál Martin, called the mounting tensions over the Falklands a “potential disaster” and said he would “probably be able to settle it out,” without providing details of what that would entail.

Burnham and Trump discussed the Falklands issue in New York last week on the sidelines of the U.N. General Assembly, but the details of the conversation have yet to be made public. TIME has reached out to the White House for comment.

Benwell says Trump’s comments, although potentially divisive, are no indication that U.S. foreign policy is set to change. “These are just statements so far, and there's been no substantive changes to U.S. foreign policy,” he emphasizes.