World leaders called for calm Thursday as Venezuela decried joint US-Guyana military exercises as a "provocation" and vowed to push ahead with the "recovery" of an oil-rich region both neighbors claim as their own.
The UN Security Council called an urgent meeting for Friday on the fast-escalating row that Guyana said "threatens international peace and security."
Fears of the conflict blowing up have deepened after Venezuelan President Nicolas Maduro's government held a controversial referendum Sunday on the fate of the Essequibo region.
Maduro's 2018 reelection was rejected by the United States and dozens of other nations as fraudulent. He is widely expected to seek a third consecutive term next year.
Addressing the recent flare-up, Guyanese Vice-President Bharrat Jagdeo said Venezuela "is not going to succeed, now or ever" at taking the Essequibo region.
"Every single movement that Venezuelans make, particularly in the proximity of our borders, is tracked, every single one of them."
The United States, through National Security Council spokesman John Kirby, urged the sides to find a diplomatic solution to the territorial dispute, saying "we don't want to see this come to blows."
But Washington provoked an angry response from Caracas by announcing via the embassy in Georgetown it would hold joint "flight operations within Guyana" as part of "routine engagement and operations to enhance security partnership" with its ally.
"This unfortunate provocation by the United States in favor… of ExxonMobil in Guyana is another step in the wrong direction," Venezuelan Defense Minister Vladimir Padrino Lopez said on X, formerly Twitter.
He added: "We will not be diverted from our future actions for the recovery of the Essequibo," where the US oil giant has discovered crude.
Essequibo has been administered by Guyana for more than a century and is the subject of border litigation before the International Court of Justice (ICJ) in The Hague.
It makes up about two-thirds of Guyanese territory and is home to 125,000 of the country's 800,000 citizens, but is also claimed by Venezuela, which does not recognize the ICJ's jurisdiction and is seeking to bring the area under its rule.
In Brazil, President Luiz Inacio Lula da Silva voiced "growing concern" about the tensions on his country's northern border.
Lula told a summit of the Mercosur South American bloc: "If there's one thing we don't want here in South America, it's war."
The regional group, composed of Argentina, Brazil, Paraguay and Uruguay, later issued a statement urging Georgetown and Caracas to seek a peaceful solution and warning against "unilateral actions." It was also signed by Chile, Colombia, Ecuador and Peru.
The Brazilian army said Wednesday it was reinforcing its presence in two northern cities.
British Foreign Secretary David Cameron also warned Venezuela on Thursday not to take "unilateral action" in the dispute. Guyana is an English-speaking former colony of Britain and the Netherlands.
Cameron spoke in Washington after a meeting with Secretary of State Antony Blinken, who in a phone call with Guyana's President Irfaan Ali on Wednesday reaffirmed the United States' "unwavering support for Guyana's sovereignty."
Asked Thursday how far Washington would go in supporting Guyana, Kirby said he did not want to "speculate about that kind of thing."
Guyana insists Essequibo's frontier was determined by an arbitration panel in 1899.
But Venezuela claims the Essequibo River to the region's east forms a natural border recognized as far back as 1777.
The long-running dispute has intensified since ExxonMobil discovered oil there in 2015, and Caracas called a referendum after Guyana started auctioning off oil blocks in August.
In Venezuela's plebiscite on Sunday, participants were asked among other things whether citizenship should be granted to the English-speaking people of a new "Guyana Esequiba State" and "consequently incorporating said State on the map of Venezuelan territory."
Officials in Caracas said 95 percent of voters supported the measures.
Two days after the vote, Maduro proposed a bill to create a Venezuelan province in Essequibo and ordered the state oil company to issue licenses for extracting crude in the region.
Ali protested a "direct threat" against his country.
Guyana's armed forces were on "alert," Ali said in a rare address to the nation late Tuesday, and in contact with "partners" including the United States.