Frankfurt - An international tribunal on Monday began hearing arguments in a dispute between Ghana and Ivory Coast over a contested maritime boundary that cuts through lucrative offshore oil fields.
The area is believed to hold the biggest hydrocarbon resources discovered in west Africa over the last decade.
The neighbours have asked the Hamburg-based International Tribunal for the Law of the Sea to rule on where the border lies exactly, with Ghana arguing there is no need to deviate from a decades-long "mutually agreed and recognised" boundary line based on equidistance.
Ivory Coast however claims the border is not properly demarcated and accuses Ghana of carrying out oil exploration activities in Ivorian maritime zones.
Both sides will lay out their case in oral hearings over the next two weeks, with Ghana kicking off the proceedings on Monday.
"The boundary lies in the region of some of the most significant oil reserves in west Africa," Ghana's justice minister Gloria Akuffo told the court in her opening statement.
"A fairly drawn agreed line does not suddenly become unfair simply because one state decides it would be economically more advantageous for it if the line were drawn somewhere else."
A tribunal spokesperson told AFP that a judgement in the case is not expected until "later this year".
The two countries brought their spat before the tribunal in 2014 after months of fruitless negotiations.
In a setback for Ghana in 2015, the court's special chamber ordered it not to carry out any new oil drilling in the disputed waters until the matter was resolved.
But the court rejected Ivory Coast's demand that Ghana suspend all ongoing oil exploration and exploitation in the area, ruling that such a move would cause the country "considerable financial loss" and could harm the maritime environment by allowing equipment to deteriorate.
Several oil companies including France's Total, Russia's Lukoil and the UK's Tullow Oil have reported significant finds in the contested area in recent years.
The dispute carries echoes of another maritime border row on the continent between the eastern nations of Kenya and Somalia.
The UN's highest court in The Hague agreed earlier this month to hear their case, which will determine the fate of offshore oil and gas projects in the Indian Ocean waters.
"The relationship between Ghana and Cote d'Ivoire remains cordial, a testament to the strength of our relationship," Akuffo told the court.
Ghana is a major producer of gold and cocoa and began commercial oil production in 2010 following the discovery of the Jubilee oil field, which produces 100 000 barrels per day.
Ivory Coast, the world's largest producer of cocoa, has a daily oil production of some 45 000 barrels and is aiming to increase its output to 200 000 barrels per day by 2020.