By Daniel Schweimler
BBC News, Buenos Aires
The environmental group Greenpeace has blocked a shipment from leaving Chile for Uruguay in the latest stage of a row over two pulp mills.
The ship had been due to sail from the Chilean port of Talcahuano
The mills are being built on the Uruguayan side of the river that separates it from Argentina.
The Uruguayan people and its government welcome the jobs and investment the project will bring.
But Argentinians say they will pollute the river and have been blocking bridges that link the two countries.
The Greenpeace team in small boats managed to stop the Baltimar Sirius from leaving the port of Talcahuano, south of the Chilean capital, Santiago.
They painted "No to the pulp mills" on the side of the vessel.
Environmentalists are fighting against the paper mills
It was carrying materials for the construction of a pulp mill in Uruguay - a project that has caused a huge rift between Uruguay and Argentina.
The Finnish company Botnia, which is building the factory on the bank of the Uruguay River that separates the two countries, says it will be perfectly safe.
Uruguay is firmly behind the project, but residents on the Argentine side of the river, backed by international environmentalists, say the plant will pollute the river.
And they have been blocking one of the main bridges linking the two countries for several weeks.
The government in Buenos Aires says it will take Uruguay to the International Court of Justice in The Hague for breaking international treaties governing the management of the waters.
President Nestor Kirchner of Argentina has called for the construction work, which is already well advanced, to be halted for 90 days to allow an independent study to be carried out.
The Uruguayan government says that that won't happen.
A small disagreement over a building project has escalated into an international incident and is driving two formerly friendly neighbours further apart.