Top US diplomat Marco Rubio bluntly warned Venezuela against attacking its oil-rich neighbour Guyana, brandishing the power of the US military to protect the small South American nation.

Venezuela, whose leftist leader Nicolas Maduro is despised by the Cuban-American Rubio, has increasingly pushed its claim to the disputed Essequibo region controlled by Guyana, and was accused this month of making an incursion.

"I have full confidence saying it now as secretary of state – there will be consequences for adventurism. There will be consequences for aggressive action," Rubio said in Guyana.

Asked what the United States would do if Venezuela attacked ExxonMobil oil projects in Guyana, Rubio said: "It would be a very bad day -- a very bad week for them."

Stopping short of spelling out a military response, Rubio said: "We have a big navy, and it can get almost anywhere."

Maduro responded angrily to the threat, calling Rubio an "imbecile" and saying "no one threatens Venezuela."

While in Guyana, Rubio signed an agreement to boost security cooperation with the country including through greater information sharing. It comes years after Guyana and the United States agreed to joint maritime patrols.

Guyanese President Irfaan Ali welcomed the stance of Rubio, who called Venezuela's claims "illegitimate."

"I'm very pleased at the reassurance of the US, ensuring the safeguard of our territorial integrity and sovereignty," Ali said.

Venezuelan Foreign Minister Yvan Gil, in a statement on Telegram, said his country did "not need nor seek conflicts, but neither will we allow foreign interests to try to rewrite the reality about our Essequibo."

"Get your noses out of this dispute!" he added.

Guyana, an English-speaking former British and Dutch colony where the majority of the 800,000 people still live in poverty, has for years had a long-shot movement that has sought to join the United States.

Such formal accession was not on the agenda, but Trump has made no secret of his passion for expansionism in the Western Hemisphere, even at the expense of traditional alliances.

While Exxon has dominated the oil industry, China has made rapid inroads in Guyana with infrastructure projects including an expansion of the international airport where Rubio landed. (AFP)