Brazil's defence ministry has "intensified" operations along the country's northern border amid a mounting territorial dispute between neighbours Venezuela and Guyana, it told AFP Friday.

Tension is running high as Venezuela prepares to hold a referendum Sunday on whether it is the rightful owner of the oil-rich Essequibo region, which makes up more than two-thirds of neighbouring Guyana.

The Brazilian defence ministry "has been following the situation. Defence operations have been intensified in the country's northern border region, leading to a larger military presence," it said in a statement sent to AFP.

The UN's top court, the International Court of Justice, ordered Venezuela on Friday to refrain from any action that would "modify the situation that currently prevails in the territory in dispute."

Guyana had asked the ICJ to call a halt to the referendum, saying it poses an "existential" threat to the country.

Guyana, which gained independence from Britain in 1966, has administered Essequibo for more than a century.

But the territory is the subject of a decades-old dispute with Venezuela.

The row intensified after ExxonMobil discovered oil in Essequibo in 2015, helping give Guyana -- population 800,000 -- the world's biggest crude reserves per capita.

The border dispute has created diplomatic awkwardness for South American giant Brazil, which borders both countries.

Brazil's top diplomat for Latin American affairs, Gisela Maria Figueiredo, said Thursday that President Luiz Inacio Lula da Silva's administration was following the situation with "concern." (AFP)