Equatorial Guinea asked judges at the International Court of Justice to reject Gabon's claim to several islands in potentially oil-rich waters in the Gulf of Guinea.
The African neighbours, both significant oil producers, have asked the United Nations' top court to settle a dispute centring on the tiny island of Mbanié, less than a kilometre long, off the coast of Gabon.
On Monday, Equatorial Guinea's representative at the International Court of Justice (ICJ), Domingo Mba Esono said: "Gabon's position is factually and legally untenable".
The conflict has been ongoing since 1972, when Gabon's army drove Equatorial Guinea soldiers from Mbanié.
Gabon has since set up its own military presence on the virtually uninhabited island of just 30 hectares.
Oil prospects
But the dispute lay dormant until the early 2000s, when the prospect of oil rekindled interest in the Gulf of Guinea.
In 2016, after years of mediation by the United Nations, the countries signed an agreement that would ultimately let the ICJ - also known as the World Court - settle the dispute.
Equatorial Guinea bases its claim on the islands on a 1900 convention dividing up French and Spanish colonial assets in West Africa.
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Gabon, meanwhile, says the ICJ should base its judgment on another agreement from 1974.
Equatorial Guinea says the document Gabon has offered as proof for the 1974 agreement is unsigned and not an original.
Hearings will last a week and Gabon will present its case on Wednesday.
The court is expected to give its final and binding ruling sometime next year.
(With wires)
