“The world has to know — the Republic of Guyana has to know,” he said, “the Esequibo is ours.”

Venezuela claims nearly three-quarters of Guyana. Guyana wants help.

The reaction has been swift. Brazilian President Luiz Inacio Lula de Silva offered Thursday to mediate between the South American neighbors. The U.N. Security Council scheduled a closed-door meeting on the matter for Friday. The U.S. Embassy in Guyana announced joint flight operations Thursday by the Guyana Defense Force and U.S. Southern Command.

Venezuela has long claimed the Esequibo, a sparsely populated region of forest, swamp and scrubland. It’s a rare point of agreement in the deeply divided country; generations of schoolchildren have been raised on maps like the one Maduro wielded during his address Tuesday.

Guyana has repeatedly rejected those claims, saying an 1899 international arbitration resolved the dispute. Venezuela has challenged the validity of that ruling. Its objections have intensified since ExxonMobil discovered massive reserves of oil in the ocean floor off the territory in 2015, a windfall that has turned Guyana, previously one of the hemisphere’s poorest countries, into one of the world’s fastest-growing economies.

The International Court of Justice — to which the United Nations, at Guyana’s urging, has referred the matter — urged both sides last week to refrain from “any action which might aggravate or extend the dispute.”

Maduro’s bluster seems unlikely to escalate to action. He and several members of his inner circle are under federal criminal indictment in the United States on charges of narcoterrorism. The once-booming oil industry — the country is home to the world’s largest proven reserves — has been crippled by outdated infrastructure, chronic mismanagement and U.S. sanctions.

Amid an economic collapse, Maduro has worked to improve relations with Washington. The Biden administration agreed in October to ease some restrictions on Venezuela’s oil sector in exchange for Maduro’s pledge to hold freer elections next year — a breakthrough between the two countries, which severed diplomatic relations in 2019.

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But his remarks on Esequibo have unsettled Guyana and drawn warnings from the United States and Brazil.

Guyanese President Irfaan Ali told CNN this week that Maduro’s declaration was a “desperate attempt by Venezuela to seize” his country’s territories. “We are taking every precautionary measure,” he said, including appeals to the United States, Brazil and the United Nations for diplomatic and military support to deter a Venezuelan invasion.

The State Department confirmed the contact. “Secretary of State Antony J. Blinken spoke with Guyanese President Dr. Mohamed Irfaan Ali to reaffirm the United States’ unwavering support for Guyana’s sovereignty,” the department said late Wednesday. Department spokesman Matthew Miller told reporters that Washington supports a peaceful resolution.

Brazil reinforced its northern border with armored vehicles and more troops, Reuters reported this week. Because Esequibo is largely inaccessible, the main road connecting Venezuela and Guyana runs through Brazil. Senior Brazilian diplomats have conveyed serious concerns to Venezuela, Reuters reported.

In a referendum on Sunday, Maduro said, more than 95 percent of Venezuelan voters expressed support for annexing Esequibo.

That percentage could reflect the actual popularity of the measure. But the government’s claim that more than 10 million Venezuelans cast ballots in the vote “makes no sense,” according to Enderson Sequera, strategic director for the Venezuela-based political analysis firm Politiks. That would be an unusually high turnout for Venezuela.

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Oil is not the only motivating factor for Maduro, Sequera said. The fixation on Esequibo also represents a sense of political insecurity following María Corina Machado’s resounding victory in the opposition’s presidential primaries in October. Machado, a longtime government critic, could present a formidable challenge Maduro in an election next year.

Now, Sequera said, “the government’s only options are to try to rile up nationalist sentiments with Guyana and gradually escalate the situation and to increase political repression and persecution.”