Original sentence: Romania and Ukraine had been negotiating a broad treaty of friendship and cooperation for several years, but disagreement over ownership of the Serpents Island and more importantly the oil and gas reserves that are thought to lie beneath them, as well as the northern border of Romania with Ukraine, had kept the sides apart
Edited sentence: Romania and Ukraine had been negotiating a broad treaty of friendship and cooperation for several years, but disagreement over ownership of the Serpents Island and more importantly the oil and gas reserves that are thought to lie beneath its area of the Black Sea, as well as the northern border of Romania with Ukraine, had kept the sides apart