Drilling by ExxonMobil last month intersected more than 90m of high-quality oil-bearing sandstone reservoirs being intersected in 1743m of water.

While ExxonMobil is still to assess the commercial viability of the discovery, and further define the potential of the entire 26,800 square kilometre block, the discovery has emboldened Venezuela to press ahead with claims the area sits within its territory.

ExxonMobil first signed an agreement with Guyana to explore the block in 1999, but the long simmering border dispute between the two nations stepped up when drilling started in March.

The discovery of a potentially lucrative oil discovery has only exacerbated tensions further.

Guyana's new government, headed by former brigadier David Granger, described the territorial decree issued by Maduro as an attempt to annex its waters.

The decree, announced a week after ExxonMobil announced its discovery, creates a "defence zone" that claims to give Venezuela access to much of Guyana offshore areas and leave the former British colony limited access to the Atlantic Ocean.

Guyana's foreign ministry describes the decree as a flagrant violation of international law.

The controversy centres on land to the west of Guyana's Essequibo River, encompassing around two-thirds of the small English-speaking nation, population 735,554.

Guyana said any attempt by Venezuela to enforce its claims will be "vigorously resisted" and brought to the attention of the international community.

Back in April, Venezuela told ExxonMobil it was drilling in the disputed territory, but the big American says it is following its host countries' and international law, and will leave border disputes to others.

In the last flare-up in 2013, Venezuela's navy evicted a ship used by Texas-based Anadarko Petroleum to explore for oil in the offshore Roraima block.

Venezuela has the world's largest oil reserves, according to OPEC, and earns 96% of foreign income from oil.

In contrast, Guyana is the third-smallest independent state on the mainland of South America after Uruguay and Suriname, and its economy relies on agriculture, mining, mining, timber and shrimp fishing.

Sugar exports comprise almost 30% of its exports.

The area has been limited exploration for hydrocarbons.

The dispute dates back to 1840, when the area was comprised of Dutch and British colonies.