Guyana has called for the immediate release of the crew of two fishing vessels it says were seized by Venezuela, less than a month after the latter unilaterally extended its maritime boundary.

It is the latest incident of a century-long border dispute between the two countries, which has heated up since US oil giant Exxon Mobil discovered crude oil in the region in 2015.

Guyana's foreign ministry said in a statement late Saturday that it had not been informed by the Venezuelan government of the detentions.

But it confirmed reports that the boats were boarded and commandeered in Guyanese waters Thursday by the crew of the Venezuelan naval vessel Comandante Hugo Chavez, named after the late Venezuelan president.

"Guyana condemns in the strongest possible terms this wanton act of aggression by the Venezuelan armed forces," the ministry said.

It said it was "currently seeking to ascertain the status and welfare of the crew members," who were being detained at Port Guiria in northeastern Venezuela.

Caracas has been pressing a historic claim to Guyana's Essequibo region -- which encompasses two-thirds of the former British colony -- after the recent oil discovery.

Guyana maintains that valid land borders were set in 1899 by an arbitration court decision in Paris, a decision Venezuela has never recognised.

In December, the UN's top court ruled it had jurisdiction in the matter -- meaning the International Court of Justice (ICJ) will now hold full hearings on the merits of the overall case, a process that could take years and which is opposed by Venezuela.

In response, President Nicolas Maduro issued a decree in January establishing a new maritime territory of Venezuela, which overlaps with Guyana's territorial waters, as well as its land territory west of the Essequibo River.

Guyana condemned the decree as a flagrant violation of its sovereignty, and of the fundamental rules of international law.

In recent years, the Venezuelan Navy has intercepted seismic research vessels collecting data for ExxonMobil and another American oil company, Anadarko Petroleum.

Caracas has condemned Guyanese oil exploration in the contested waters, as well as recent joint US-Guyana military exercises in the area.

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ANADARKO PETROLEUM