Brazil has deployed troops to its border with Venezuela after Caracas announced plans to make an oil-rich region of Guyana its own.

Nicolas Maduro, the Venezuelan president (left), held a referendum on Sunday, over the sovereignty of the Essequibo region.

The move came as a military helicopter carrying seven people vanished near Guyana’s border with Venezuela on Wednesday.

Authorities said there was bad weather in the area and stressed there was no indication it may have been hit by hostile fire as tensions escalate between the countries.

Venezuela has become increasingly hawkish in pushing its claim over Essequibo – which makes up two-thirds of Guyana’s territory and is home to around a sixth of its population – since vast oil reserves were discovered off its coast eight years ago.

On Sunday, Nicolás Maduro, the Venezuelan president, held a referendum in which an overwhelming majority of voters backed the country’s claim of sovereignty over the region.

Guyana dubbed the vote, which had a turnout of about 50 per cent, a “pretext for annexation” by its neighbour.

Two days later, Mr Maduro said he would immediately grant operating licences for exploration and exploitation in Essequibo and proposed that the National Assembly pass a bill to make the area part of Venezuela.

The Brazilian army said on Wednesday that it was reinforcing its presence in the northern cities of Pacaraima and Boa Vista near its frontier with Venezuela as part of efforts “to guarantee the inviolability of the territory”.

It said an infantry brigade with almost two thousand soldiers had “intensified” a military presence in the border region.

The troops are tasked with “surveillance and protection of the national territory”, the army said, adding that armoured vehicles will be sent from Brazil’s southern and central regions to reinforce the deployment.

The United States said it was closely monitoring the situation.

John Kirby, the National Security Council spokesman told reporters: “It’s concerning. We’re watching this very, very, very closely.

“We obviously don’t want to see any violence occur here or conflict occur,” he added.

Guyana and Venezuela meanwhile agreed to keep “communication channels open” amid the fast-escalating feud.

In Caracas, the government said Yvan Gil, its leading diplomat, had a phone call with Hugh Todd, his Guyanese counterpart, to “discuss the issue of the territorial dispute”.

The discussion took place at Guyana’s request, Venezuela said, and concluded with the parties agreeing to “keep the communication channels open”.

Irfaan Ali, the Guyanese president, has called Mr Maduro’s moves, which include issuing an ultimatum to oil companies working under concessions issued by Guyana to halt operations within three months, a “direct threat” against his country.

Guyana’s armed forces were on “alert”, Mr Ali added in a rare address to the nation late on Tuesday, and were in contact with “partners” including the United States.

Essequibo is home to 125,000 of Guyana’s 800,000 citizens.

A long disputed territory

Litigation is pending before the International Court of Justice (ICJ) in The Hague over where the region’s borders should lie.

Last week, two days before the referendum, the ICJ ordered Venezuela to “refrain from taking any action which would modify the situation that currently prevails in the territory in dispute”.

It did not, however, grant an urgent request by Guyana to stop Sunday’s referendum.

Guyana, a former British and Dutch colony, insists the Essequibo frontiers were determined by an arbitration panel in 1899.

But Venezuela - which does not accept the ICJ’s jurisdiction in the matter – claims the Essequibo River to the region’s east forms a natural border between the two countries, as declared in 1777 under Spanish rule, and that Britain wrongly appropriated Venezuelan lands in the 19th century.

Caracas called a referendum after Guyana began auctioning off oil blocks in Essequibo in August.

Venezuela’s position is that a 1966 agreement between Venezuela and then colonial ruler Britain nullified the 1899 ruling.

Venezuelan voters were asked in Sunday’s referendum to respond to five questions, including whether Caracas should reject the 1899 arbitration decision as well as the ICJ’s jurisdiction.

They were also asked whether Venezuelan citizenship should be granted to the people – currently Guyanese – of a new “Guyana Esequiba State”.

Officials said 95 per cent of voters supported the measures.