Bolivia has filed a lawsuit against Chile at the International Court of Justice in The Hague to reclaim access to the Pacific Ocean.
Bolivia lost access to the coastline in a 19th Century war with Chile, leaving it landlocked ever since.
Chile says Bolivia's demand has no historical or legal basis.
The two countries have had limited diplomatic relations since 1978, and previous attempts to negotiate the redrawing of the border have failed.
Speaking in The Hague, Bolivian Foreign Minister David Choquehuanca said his nation had demanded that Chile negotiate in good faith with Bolivia "a swift and effective agreement that grants it [Bolivia] fully sovereign access to the Pacific Ocean".
Bolivian President Evo Morales said he had decided to file the suit after "listening to the Bolivian people".
He had earlier expressed his confidence that Bolivia would regain its access to the sea, saying: "We're going to win this battle because we're right."
Reacting to the news, Chilean Interior Minister Andres Chadwick ruled out any dialogue.
"If they want to talk about Chile's maritime sovereignty, no. No dialogue is possible," he said.
The borders between the two neighbours date back to the 1904 Treaty of Peace and Friendship, signed after Bolivia lost 400km (250 miles) of coastline to Chile during the War of the Pacific (1879-1884).
Bolivia has long tried to reclaim the territory and has repeatedly severed relations with Chile when those attempts failed.
It still maintains a small navy and each year celebrates the Day of the Sea.
President Morales had announced his intention to take the case to the International Court of Justice at this year's celebrations to mark the day.
The Chilean government said it was not worried about Bolivia's move, as "no-one in the world will accept that a country unilaterally dismisses a treaty which is in full force".