Guyana's new government attacked on Monday a territorial decree by Venezuelan President Nicolas Maduro as an attempt to annex its waters following an oil discovery.
The dispute between the South American neighbors goes back to the early 19th century and resurfaced after an offshore oil discovery by ExxonMobil Corp last month.
The decree creates a theoretical "defense" zone offshore that would, in Venezuela's eyes, leave the former British colony with no direct access to the Atlantic.
Guyana's foreign ministry described the decree as a flagrant violation of international law.
"Guyana rejects this illegality, which seeks to undermine our development through the exploitation of our natural resources offshore," it said in a statement.
In April, Venezuelan Foreign Minister Delcy Rodriguez wrote a letter to Exxon's Guyana country manager, Jeff Simon, saying Venezuela would not accept the incursion or interference of any multinational company in the disputed territory.
The controversy centers on land to the west of Guyana's Essequibo River, encompassing around two-thirds of the small English-speaking nation on the shoulder of South America.
Granger: "Our Territory"
Maduro's decree alters the more conciliatory stance towards Guyana taken by his predecessor, Hugo Chavez, who was friendly with the previous government and sold Guyana fuel on advantageous terms under the Petrocaribe initiative.
However, elections last month unseated the party that had run Guyana for 23 years and gave former brigadier David Granger the presidency.
"I am convinced that the site which they [Exxon] are drilling is well within our exclusive economic zone," Granger told Reuters days after his win. "It's our territory. I don't see that Venezuela, of all the countries on the continent, should oppose the extraction of petroleum from one of our sites."
Venezuela has the world's largest oil reserves, according to OPEC, and earns 96 percent of foreign income from oil.
Exxon, which is drilling in the so-called Stabroek Block, about 190 kilometers (120 miles) off Guyana's coast, said its policy is to follow host countries' and international law, and that border disputes are a matter for governments.
"We are operating the Stabroek Block offshore Guyana under license from the government of Guyana," spokeswoman Lauren Kerr added in an email.
In the last flare-up in 2013, Venezuela's navy evicted a ship used by Texas-based Anadarko Petroleum to explore for oil in the offshore Roraima block.
The land in dispute has long been denoted on Venezuelan maps as a "reclamation zone", while in practice Guyanese have long lived and mined there. Beneath its jungle and savannah lie gold, diamonds and bauxite, staples of Guyana's economy.