The prime minister of Equatorial Guinea, Candido Muatetema Rivas, has accused Gabon of illegally occupying a tiny island off the coast of both countries.
Mr Rivas asked Gabon to withdraw its paramilitary police from the isle of Mbagne, which is located in potentially oil-rich waters in the bay of Corisco.
The foreign minister of Equatorial Guinea, Pastor Micha Ondo, is in the Gabonese capital, Libreville, to hand over a message to President Omar Bongo.
No details have been given about the message.
Equatorial Guinea and Gabon, both oil rich states, have both been claiming sovereignty over the islands in the bay of Corisco for the last three decades.
In 1970, Gabon sent troops to occupy the island.
The prime minister said in a statement read out on national television: "The government of the Republic of Equatorial Guinea has claimed the island of Mbagne as part of its national territory from colonial times until now."
In February, Ali Bongo, the son of the Gabonese president took a group of people to the isle of Mbagne as "a reaffirmation of our sovereignty over the strategic part of our territory".
He was accompanied by Gabon's army and navy chiefs as well as the commander in chief of the gendarmerie or military police.
Mr Rivas said the visit was "untimely".
A small but well trained gendarmerie are reported to be on the isle Mbagne.
In 1972, the isle of Mbagne was described in a document, known as the Brazzaville Communique as a "neutral zone".
It resulted from a meeting held in Congo Brazzaville which had called on both sides to settle their differences amicably.
Both Equatorial Guinea and Gabon are lead by long serving presidents with a tight grip on power.
Since the discovery of large oil and gas deposits off Bioko in the mid-1990s, Equatorial Guinea has experienced an economic boom.
Gabon has the highest income per capita in the region thanks to its oil wealth, which accounts for 80% of exports.