The United Nations' top court will decide on Friday whether to order Venezuela to halt a planned referendum that could lead to Caracas annexing a disputed oil-rich region administered by Guyana.

Venezuela has for decades laid claim to the Essequibo region, which is governed by Guyana and makes up more than two-thirds of its territory, home to 125,000 of its 800,000 citizens.

Caracas plans a referendum on 3 December calling into question an 1899 decision by an arbitration tribunal that fixed the border with Guyana — a former colony of both Britain and the Netherlands.

Guyana says the referendum poses an "existential" threat to the country and has petitioned the International Court of Justice to urgently call a halt to the vote "in its present form."

The dispute has intensified since ExxonMobil discovered oil in Essequibo in 2015.

In addition, a "significant" new oil discovery was made in the region in October, adding to Guyana's reserves of at least 10 billion barrels — more than Kuwait or the United Arab Emirates.

Tiny Guyana has the world's biggest reserves of crude per capita, while its neighbor sits on the largest proven reserves overall.

In the referendum, Venezuelans will be asked whether or not to reject the 1899 decision, which Caracas says was "fraudulently imposed."

Also on the ballot is whether Venezuela should reject ICJ jurisdiction over the dispute, and whether or not to grant Venezuelan citizenship to the people of an annexed Essequibo.