The conference determined that , as of the late 19th century , the Bulgarian ethnic territories within the Ottoman Empire extended to the Danube Delta in the northeast , Kastoria in the southwest , Kirklareli and Edirne in the southeast , and NiÅ in the northwest . The conference determined that , as of the late 19th century , the Bulgarian ethnic territories within the Ottoman Empire extended to Tulcea and the Danube Delta in the northeast , Ohrid and Kastoria in the southwest , Kirklareli and Edirne in the southeast , and Leskovac and NiÅ in the northwest .