GEORGETOWN, Guyana (AFP) — The United States on Saturday denounced what it said were Venezuelan naval vessels “threatening” an ExxonMobil unit in maritime territory claimed by Guyana.

Guyana’s president has denounced an incursion by an armed Venezuelan naval vessel in disputed waters and home to a mammoth offshore oil deposit being developed by ExxonMobil.

A diplomatic spat is brewing after Venezuelan naval vessels entered ExxonMobil’s Stabroek block offshore Guyana, reportedly threatening its floating production, storage and offloading vessels operating there including the Prosperity FPSO amid an escalating territorial dispute between the two nations.

Guyanese President Irfaan Ali said on Saturday that a Venezuelan coast guard patrol entered Guyanese waters earlier in the day, approaching an output vessel in an offshore oil block managed by Exxon Mobil .

General, the Rt Hon Patricia Scotland KC, has echoed the alarm of Dr Irfaan Ali, the President of the Co-operative Republic of Guyana, after a coast guard vessel from the Bolivarian Republic of Venezuela entered the waters of Guyana on 1.

The United States issued a stern warning to the Nicolás Maduro regime on Saturday following reports that a Venezuelan patrol boat entered into Guyanaese waters, threatening ExxonMobil’s offshore operations.

The South American neighbours are involved in a long-running dispute about which country owns the oil-rich Essequibo area.

Guyana and Venezuela clashed again after the former complained of incursion into its oil-rich maritime territory, which then triggered a warning from Caracas against “dangerous provocation” based on “lies”.

Guyana’s government said Saturday that a Venezuelan naval patrol ship entered its waters and approached a vessel contracted by Exxon Mobil (NYSE:XOM) in the Straboek block, according to media reports.