Georgetown (Guyana), Mar 27 (AP) US Secretary of State Marco Rubio on Thursday vowed that the United States will respond forcefully if Venezuela attacks Guyana amid an ongoing territorial dispute that includes massive oil and gas reserves.

Rubio said it would be a "very bad day" for Venezuela should that happen.

On the second leg of a three-nation tour of the Caribbean, Rubio was in Guyana as the Doland Trump administration ramps up engagement in the Western Hemisphere to promote energy independence and curb illegal migration, drug trafficking and gang violence.

He made a brief stop in Georgetown on Thursday for talks with Guyanese President Irfaan Ali and other officials before travelling to Suriname for an even shorter visit to Paramaribo.

"The regional threats are based on illegitimate territorial claims by a narco-trafficking regime," Rubio told reporters at a joint news conference with Ali.

"And I want to be frank, there will be consequences for adventurism. There will be consequences for aggressive actions. It would be a very bad day for the Venezuelan regime if they were to attack Guyana or attack ExxonMobil or anything like it," he added.

“It would be a very bad day or a very bad week for them, and it would not end well for them." Ali said his government, which has vehemently condemned Venezuelan actions, including earlier this month, welcomed Rubio's words.

"I am very pleased at the reassurance of the US in ensuring the safeguard of our territorial integrity and sovereignty. The threats from Venezuela were specifically discussed," he said.

Guyana, which has significant offshore oil deposits, will play a key role in the Republican administration's attempts to wean the region from imports of Venezuelan energy, against which President Donald Trump has imposed major sanctions.

It also has a troubled relationship with Venezuela.

Earlier this month, it denounced an incursion by an armed Venezuelan naval vessel in disputed waters where ExxonMobil is developing a mammoth oil deposit.

Ali said on March 1 that the Venezuelan ship passed close to several huge vessels used to store oil being produced by three oilfields operated by Exxon and partners Hess Corp and CNOOC of China in the Stabroek Block, which contains an estimated 11 billion barrels of oil.

Since then, the Trump administration announced severe sanctions, including 25 per cent tariffs on goods produced in countries that import Venezuelan oil.

Rubio signed a security agreement with Guyana's foreign minister, according to the US state department, that will enhance intelligence sharing and military-to-military cooperation. The signing came as the US Navy was conducting exercises with the Guyanese Navy in its exclusive economic zone.

Since ExxonMobil made its major oil discovery in Guyana in 2015, Venezuela has revived a century-old territorial dispute with Guyana and taken steps to annex the remote Essequibo region, which comprises about two-thirds of Guyana's land mass.

Maduro's government has called Guyana's development of the oil fields "illegal".

Guyana's government Thursday also announced the start of joint naval exercises with the US Navy.

Guyana said the exercises, which began this week, will involve the US Navy cruiser USS Normandy and local vessels and are aimed at “safeguarding Guyana's maritime sovereignty and enhancing regional security through collaborative training".

"This engagement also reflects the strong and growing military-to-military relationship between Guyana and the United States," Guyana's military said. (AP) ARI

Territorial sovereignty deterrence: US signals forceful response and boosts military cooperation to protect Guyana and offshore energy. The United States warned of forceful responses to Venezuelan aggression in the revived territorial dispute with Guyana over the Essequibo region and offshore oil developments, framing such acts as illegitimate and linking protection of territorial sovereignty and energy infrastructure to deterrence. The US signed a security agreement to expand intelligence sharing and military-to-military cooperation and began joint naval exercises in Guyana's exclusive economic zone to defend maritime sovereignty and safeguard offshore operations.