There's a border somewhere in the vast no-man's land of jungles and rivers between Venezuela and Guyana, but for more than a century the two countries have not been able to agree where it is.

The dispute flares up periodically whenever there's something at stake. Only now it's extending out into the ocean.

That's where Exxon Mobil Corp., drilling with a license from Guyana, said it made a significant petroleum discovery last month, 120 miles offshore in an area known as the Stabroek Block.

Venezuela, which has the world's largest proven petroleum reserves, promptly reasserted its claim on the area, in the form of a May 27 territorial decree by President Nicolas Maduro, whose government is in the throes of a deep financial crisis.

The decree rests on Venezuela's long-standing claim to the disputed area on shore, which is equal to roughly two-thirds of Guyana-controlled territory. It encompasses the resource-rich forests and savannas west of the Essequibo River, the largest waterway between South America's mighty Amazon and Orinoco rivers.

Maduro's decree essentially projects Venezuela's claim out into the Atlantic Ocean, calling the area an "operational zone for maritime defense."

On Monday, the clash intensified when the government of newly elected President David Granger issued a statement calling Venezuela's claim a "threat to regional peace and security" and a "flagrant violation of international law."

Guyana's Foreign Ministry warned that any attempt by Venezuela to enforce its claim would be "vigorously resisted."

"It is international law that must reign supreme and not the ambitions of a larger State which wishes to trample upon the rights of a smaller country in order to obstruct the sovereign right of Guyana to develop its natural resources," the statement said.

The border disagreement between the two countries stretches back to an 1899 court ruling. At the time, the United States backed Venezuela's claims against Guyana, a former British colony. Venezuela disavowed the ruling in 1962 and has challenged its validity ever since.

Before his death in 2013, the late Venezuela leader Hugo Chavez alleviated the standoff with shipments of subsidized oil to Guyana's left-leaning government.

But the election of Granger, a former army general, unseated Guyana's long-ruling People's Progressive Party and has also shaken up relations with Venezuela, which gets 96 percent of its export earnings from oil.

Venezuela's Foreign Ministry has not tried to interfere with Exxon Mobil's operations in the area, but it has repeatedly warned the company that it does not recognize the drilling license issued by the Guyanese government.

In 2013, Venezuela's navy detained a ship and drilling crew working for Texas-based Anadarko Petroleum Corp. in another disputed offshore area.

Venezuela and Exxon Mobil have a long-standing dispute of their own. Last year, an international arbitration court ruled that Caracas owes the company $1.6 billion in compensation for properties nationalized by Chavez in 2007.