THE HAGUE, September 2, 2008 (AFP) - A decades-long dispute between Romania and Ukraine over a zone of the Black Sea said to contain hydrocarbon deposits entered a new phase Tuesday when it came before the UN's highest court in The Hague.

Romania fired the opening volley at the start of two weeks of public hearings, accusing Ukraine of having shown bad faith in six years of failed bilateral negotiations aimed at drawing up maritime boundaries.

'It is not the sophisticated nature of this case but the non-compliance of Ukraine with the rules of delimitation that led to the failure of the bilateral negotiations,' foreign affairs official Bogdan Aurescu argued for Romania before a panel of 16 judge of the International Court of Justice.

The dispute concerns an area of about 12,400 square kilometers (4,800 square miles) and plans by Kiev to create an exclusive economic zone (EEZ) around the small Serpents Island where natural gas and oil deposits are thought to be concentrated.

The uninhabited 17 hectare (42 acre) islet has been described as 'a tiny little rock' by Romania which owned it until 1948 when it was ceded to the then Soviet Union.

Romanian experts have estimated the area around the island may contain 100 billion cubic metres of gas.

Romania instituted proceedings against its neighbour in September 2004 after six years of dedicated bilateral negotiations failed to resolve the matter -- a cause of rivalry between the states for decades.

Aurescu told the judges on Tuesday that Ukraine was seeking a 'profoundly inequitable result' based on 'flawed theory'.

And he argued that maritime boundaries in the area had been determined in agreements between Romania and the former Soviet Union, which remained in place.

While The Serpents Island was nothing but a 'rock' with no vegetation or water, the registration of an EEZ around it would rob Romania of valuable maritime territory, Aurescu argued.

A court statement said the hearing concerned 'the establishment of a single maritime boundary between the two states in the Black Sea, thereby delimiting the continental shelf and the exclusive economic zones appertaining to them'.

The ICJ is the highest court of the United Nations, set up to rule on disputes between states. Cases can take years to be concluded.

Aurescu said his country welcomed the chance of a court-determined settlement.

'For a European country the size of Romania, international law is the most effective instrument for settling international disputes,' he told the judges.

Romania will present arguments for the remainder of the week, followed by the Ukraine next week. The case has been set down until September 19.