Venezuela’s government said voters approved a plan to annex the oil and gas-rich Essequibo region of neighboring Guyana in a referendum held Sunday.

Essequibo contains the offshore Stabroek block, where an ExxonMobil-led consortium has made numerous oil discoveries in recent years. 

The region, which was part of Venezuela during the Spanish colonial period, has been a point of contention for Venezuelans since the 19th century, when Guyana – then a British colony – declared Essequibo as its own.

An international arbitration panel upheld Guyana’s claim to the territory in 1899, a decision that Venezuela has periodically contested through various channels. 

The International Court of Justice in The Hague warned Venezuela on Friday (Dec. 1) not to proceed with the...