CARACAS, Venezuela, (AFP) — Leading Venezuelan opposition figure Maria Corina Machado called for authorities on Wednesday to suspend a controversial referendum to annex a disputed oil-rich territory administered by neighbouring Guyana.

Machado, who won the opposition primary ahead of next year’s presidential election, called the annexation bid a “distraction” from Venezuela’s ongoing political and economic crises.

The government of Nicolas Maduro has organized a December 3 poll to ask Venezuelans to consider annexing the Essequibo region, which makes up two-thirds of tiny Guyana but is claimed by Caracas.

“It is a mistake that not only does not contribute to the defence of our territory, but it can also harm us before our defence in the International Court of Justice,” Machado said in a statement on social media.

“The referendum on Essequibo must be suspended.”

The Maduro government rejected the outcome of the opposition primary won by Machado, 56, who had previously been banned from holding office. It remains unclear if she will remain a candidate in next year’s election.

The dispute over the 160,000-square-kilometre (62,000-square-mile) region has intensified since ExxonMobil discovered oil in Essequibo in 2015.

Another major discovery there in October added further to Guyana’s reserves, making them greater than those of Kuwait or the United Arab Emirates.

The dispute over Essequibo dates back to 1899 when an arbitration tribunal fixed the border between Venezuela and Guyana — a former colony of both Britain and the Netherlands.

Venezuela maintains that the Essequibo River to the east of the region forms a natural frontier, recognised at the time of independence from Spain.

Venezuelans will be asked whether to reject the 1899 tribunal decision, which Caracas says was “fraudulently imposed.”

Also on the ballot is whether Venezuela should reject International Court of Justice jurisdiction over the dispute, and whether to grant Venezuelan citizenship to the people of an annexed Essequibo.

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