Brazil rejects any use of force by Venezuela to occupy the Esequibo territory in Guyana and will urge the Caracas government not to threaten its neighbor, President Luiz Inacio Lula da Silva's top foreign policy advisor said on Wednesday.

Celso Amorim, who traveled to Venezuela as Lula's special envoy last month, told Reuters that Brazil opposes "the use of force or threat thereof."

"I conveyed our very serious preoccupations," he said of his visit to Caracas ahead of a public referendum on the Esequibo.

"Now there are new facts that are still more worrisome. We'll not fail to transmit our concerns especially in relation to the policy of no use of force," Amorim said.

Venezuelans voted in a referendum on Sunday to approve the annexation of the Esequibo. The Venezuelan government held the vote despite an International Court of Justice ruling barring Venezuela from taking any action that would change the status quo in the disputed area.

Brazilian Foreign Minister Mauro Vieira, speaking to Reuters in Rio de Janeiro on Wednesday, dismissed the risk of an armed clash between Brazil's two neighbors on the northern border.

However, Brazil's military has reinforced the border region due to rising tensions between Venezuela and Guyana. The Brazilian army is moving armored vehicles and more troops to Boa Vista, the capital of Roraima state, the Defense Ministry said.

At issue is a 160,000 square km (61,776 square mile) region more than twice the size of Ireland that is mostly thick jungle. Venezuela reactivated its claim over the Esequibo in recent years after the discovery of vast offshore oil and gas reserves.

The Esequibo has been in dispute since the 19th century when Guyana was a British colony. An international tribunal in Paris settled the issue in 1899, but Venezuela says the decision was rigged.