Greek authorities rescued 91 migrants from a river islet and transferred them to a processing center near the border with Turkey, police said Sunday. The migrants had been on an islet on Evros River at least since Friday, when nongovernmental organizations emailed Greek authorities alerting them to their presence there. By early Sunday, the migrants had moved to the Greek side, enabling the rescue operation, a statement from the prime minister’s office said. Almost all of the land border between Greece and Turkey is formed by the Evros River, called Meric in Turkey. Greece has built a high fence along much of the border to prevent migrants crossing, and is planning to further extend it.