Equatorial Guinea has urged the ICJ to dismiss Gabon's claim to several islands located in the oil-rich waters of the Gulf of Guinea.
Both nations have brought the matter to the United Nations' top court to resolve a dispute over the island of Mbanie, which is less than a kilometer long and lies off Gabon's coast in the oil-rich waters.
"Gabon's position is factually and legally untenable," said Equatorial Guinea's representative at the court, Domingo Mba Esono.
The conflict began in 1972 when Gabon's military expelled Equatorial Guinean soldiers from Mbanie. Since then, Gabon has maintained a military presence on the largely uninhabited island. The dispute lay dormant until the early 2000s, when the possibility of oil reserves rekindled interest in the region.
In 2016, following years of UN-mediated negotiations, both countries agreed to let the ICJ settle the issue. Equatorial Guinea bases its claim on a 1900 convention that allocated French and Spanish colonial assets in West Africa. Conversely, Gabon argues that the ICJ should rely on a 1974 agreement, although Equatorial Guinea insists the document Gabon presented is unsigned and not original.
The hearings are scheduled to last a week, with Gabon set to present its arguments on Wednesday.