The Nicobar Islands are an archipelagic island chain in the eastern Indian Ocean. They are located in Southeast Asia, 150 kilometres (93 mi) northwest of Aceh on Sumatra, and separated from Thailand to the east by the Andaman Sea. Located 1,300 kilometres (810 mi) southeast of the Indian subcontinent, across the Bay of Bengal, they are part of India, as the Nicobar district within the union territory of the Andaman and Nicobar Islands. Together with the Andaman Islands to their north, the Nicobar Islands serve as a maritime boundary between the Bay of Bengal to the west and the Andaman Sea to the east.