BOGOTA, Colombia (AP) — Colombian President Gustavo Petro stirred up a decades-old border controversy with Peru on Tuesday when he accused it of fully annexing an Amazon river island that has been administered by Peru for decades, but whose legal status is in dispute. Peru maintains it owns Santa Rosa Island based on treaties about a century old, but Colombia disputes that ownership because the island had not yet emerged from the river at the time. In a message on X, Petro said that Peru acted "unilaterally" in June when its congress passed a law that upgraded Santa Rosa's legal status by converting it into a district within Peru's Loreto province. The island faces Leticia, a Colombian city of about 60,000 people located in one of the most well-preserved swaths of the Amazon. It is used by many tourists as the launching point for trips into the world's largest rainforest. "The Peruvian government has just appropriated it by law," Petro wrote on X, adding that Peru's actions could block Leticia's access to the Amazon river. "Our government will resort to diplomacy to defend our national sovereignty." In his message, the president was explaining why he plans to hold a celebration in Leticia on Thursday to mark one of the national holidays that commemorates Colombia's independence from Spain. The Colombian goverment usually celebrates the Aug. 7 holiday in the province of Boyaca in central Colombia, but farmers are currently blocking roads in that part of the country to protest environmental regulations that prohibit agriculture in high-altitude areas. Peru's Foreign Affairs Ministry said in a statement that treaties signed by both countries in 1922 and 1929 granted Peru control of Santa Rosa and other nearby islets. "Peru is complying firmly with its obligations under international law and with valid bilaterial treaties" the statement said. Colombia says that the treaties could not assign ownership of Santa Rosa because in the 1920s the island had not yet emerged from the world's largest river. Instead, Colombia says that the treaties determine that the border between both countries should be set along a line that follows the deepest points along the river bed. Like many rivers, the Amazon changes its course slowly over time, and erosion or changes in the weather can create or submerge islands. Colombia's Foreign Affairs Ministry said Tuesday that "for years" it has insisted on the need to create a bilateral commission that will assign ownership of islands that have emerged between the South American countries over the past century. The island is made up mostly of forest, farmland and a small village known as Santa Rosa de Yavari, which has a population of less than 1,000 people, according to Peru's latest census. Santa Rosa was previously classified as a community within the Yavari District of Peru's Loreto Province. In June, Peru's congress voted to turn Santa Rosa into its own district, a move that could facilitate the transfer of funds for education and healthcare, and also enable the modest village to raise its own property taxes. "We have a diverse economy that relies on commerce and tourism" Santa Rosa's Mayor Jack Yovera told Peruvian network RPP in June, when the law to turn his community into a district was being debated by congress. "But there are many basic needs that have not been met" said Yovera, who explained that as a young man he had to go to high school in the Colombian city of Leticia, on the other side of the Amazon River, due to the lack of a proper high school in Santa Rosa. The border dispute also surfaced last summer, when an official from Colombia's Foreign Affairs Ministry attended a meeting between political leaders from Leticia and Santa Rosa and said that Peru had occupied the island "irregularly." Yovera left the meeting in protest. Later, Colombia's Foreign Affairs ministry issued a statement saying it "regretted" the incident and that the situation of Santa Rosa Island should be discussed only by high-level officials from both governments.