The leaders of Guyana and Venezuela were heading for a meeting on Thursday as regional nations sought to defuse a long-standing territorial dispute.

The meeting aims to ease tensions over Essequibo, a vast border region rich in oil and minerals that represents much of Guyana’s territory but which Venezuela voted in a referendum to claim as its own.

Pushed by regional partners, Guyanan President Irfaan Ali and Venezuelan President Nicolas Maduro agreed to meet on the Caribbean island of St Vincent. The prime ministers of Barbados, Dominica and Trinidad and Tobago said they also would attend.

Venezuela’s president followed the referendum by ordering his state-owned companies to explore and exploit the oil, gas and mines in Essequibo. Both sides have put their militaries on alert.

Venezuela insists the Essequibo region was part of its territory during the Spanish colonial period and argues the 1966 Geneva Agreement between their country, Britain and Guyana, the former colony of British Guiana, nullified the border drawn in 1899 by international arbitrators.

In a letter sent on Tuesday to Ralph Gonsalves, prime minister of St Vincent and the Grenadines, Guyana’s president said the Geneva Agreement states that the International Court of Justice should settle any border controversy.

“We are firm on this matter, and it will not be open for discussion,” Mr Ali wrote on X, formerly known as Twitter.

Mr Ali also said he was concerned about what he described as “inaccurate assertions” made by Mr Maduro’s own letter to Gonsalves.

He disputed Mr Maduro’s description of oil concessions granted by Guyana as being “in a maritime area yet to be delimited,” saying all oil blocks “are located well within Guyanese waters under international law, including the United Nations Convention on the Law of the Sea”.

The president also rejected what he said his neighbouring leading described as “meddling of the United States Southern Command, which has begun operations in the disputed territory.”

“Any allegation that a military operation aimed at Venezuela exists in any part of Guyanese territory is false, misleading and provocative,” Mr Ali said in his letter to Mr Gonsalves.

Mr Maduro’s letter to Mr Gonsalves repeats Venezuela’s contention that the border drawn in 1899 was “the result of a scheme” between the US and the UK and said the dispute “must be amicably resolved in a matter acceptable to both parties”.

The meeting between the two leaders was scheduled to last one day, although many expect the disagreement to drag on into next year.