Complex sentence: Since the South Korean government sent their South Korean Coast Guard to the islets, civilian travel was subject to South Korean government approval; they have claimed that the reason for this is that the islet group is designated as a nature reserve.
Split into simpler sentences: Since the South Korean Government sent their Coast Guard to the islets, civilian travel was subject to Korean government approval. They have claimed that the reason is that the islet group is designated as a nature reserve.