GEORGETOWN, Guyana, (AFP) — Guyana has asked the United Nation’s highest court to stop neighbour Venezuela from holding a referendum on whether or not to annex the disputed, oil-rich, Essequibo region, a court statement said Tuesday.

Venezuela has for decades argued that the 160,000-square-kilometre (62,000-square-mile) region administered by Guyana should fall within its borders.

The dispute, which is before the International Court of Justice (ICJ) in The Hague, intensified after ExxonMobil discovered oil there in 2015.

Guyana, much smaller than its oil-rich neighbour, has the world’s biggest reserves of crude per capita, while Venezuela sits on the largest proven reserves overall.

As the squabble intensifies, Venezuela recently announced it would hold a referendum on the issue on December 3, a move Guyana described as illegal.

On Tuesday, the ICJ said Guyana had asked it to order Venezuela “not to proceed” with the plebiscite in its current form.

In a request filed Monday, Guyana argued the referendum’s only purpose was to “obtain responses that would support Venezuela’s decision to abandon” the ICJ proceedings and allow for its “formally annexing and integrating” Essequibo into Venezuela.

The Essequibo region makes up more than two-thirds of Guyana and is home to 125,000 of its 800,000 residents, according to a decade-old census.

A former Dutch and British colony, Guyana says its border with Venezuela was fixed by an arbitration tribunal in 1899.

But Venezuela says the Essequibo river to the east of the region forms a natural frontier recognised at the time of independence from Spain.

In its filing, Guyana said the phrasing of the referendum sought to encourage citizens to reject the 1899 arbitration award as well as the ICJ’s jurisdiction in the case, and to approve the annexation of the region.

The court statement did not specify when it was likely to rule on Guyana’s request to intervene.

Venezuela reacted by saying its neighbor’s petition “if it weren’t so tragic, would be laughable.”

“What Guyana has asked is an outburst — asking Venezuela to repeal its constitutional order, which is not going to happen,” said Vice President Delcy Rodriguez.

Guyana last week announced a “significant” new oil discovery in Essequibo and said it awarded bids to eight companies to drill for crude in its waters.