ALGIERS (Reuters) – Western Sahara’s Polisario independence movement said it will ask EU and French authorities to seize the cargo of a ship it accused of illegally transporting marine oil from the Moroccan-controlled part of the disputed territory.

The case could break new legal ground in the long-running conflict over the desert region, where Polisario has declared an independent state, but which has been claimed by Morocco as part of its kingdom.

Mhamed Khadad, Polisario’s secretary for foreign affairs, said the oil shipment violated a ruling from the European Court of Justice last month that, for the purposes of two trade deals between the European Union and Morocco, said the territory of the latter did not include Western Sahara.

He said as an “occupying force”, Morocco had no right to issue export licences. The Moroccan foreign ministry declined to make any immediate comment.

Polisario said on its Sahara Press Service that it would file its complaint with the European Commission and French customs, “denouncing the illegal shipment of marine oil by a European tanker, Key Bay, from occupied Western Sahara’s town”, referring to Laayoune in the Moroccan-controlled area.

Western Sahara, which has significant phosphate reserves and offshore fishing, has been contested since 1975 when Spain, the former colonial power, withdrew. Morocco fought a 16-year war with Polisario, which established a self-declared Sahrawi Arab Democratic Republic.

Responding to an escalation in tension, U.N. peacekeeping observers have been deployed since August between Moroccan Royal Gendarmerie personnel and a unit of Polisario fighters facing off in a narrow strip of buffer zone between the two sides.

(Reporting by Patrick Markey; Editing by Mark Trevelyan)