(Adds details on ship, U.N. plan for self-determination vote)
ALGIERS, Jan 18 (Reuters) - Western Sahara's Polisario independence movement says it will ask EU and French authorities to seize a cargo that was loaded onto a tanker this month in the Moroccan-controlled part of the disputed desert territory.
The case could break new legal ground in the long-running conflict over Western Sahara, where Polisario has declared an independent state, but which has been claimed by Morocco as part of its kingdom.
Polisario said on its Sahara Press Service that it would file its complaint with the European Commission and French customs, denouncing what it called the illegal shipment by the vessel Key Bay. It described the cargo in English as marine oil, but shipping data showed it was fish oil.
Shipping databases showed the vessel's manager as Norwegian-based Sea Tank Chartering, which did not immediately respond to a request for comment.
Mhamed Khadad, Polisario's secretary for foreign affairs, said the 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 that, as an "occupying force", Morocco had no right to issue export licences. The Moroccan Foreign Ministry declined to make any immediate comment.
The 4,570 deadweight tonnes Gibraltar-flagged tanker Key Bay made a port call to Moroccan-controlled Laayoune on Jan. 5 before sailing on to Nouadhibou in Mauritania between Jan. 10-14, according to ship tracking data on Reuters.
The vessel's last reported position was off the coast of Portugal at 1615 GMT on Wednesday, showing a destination of Fecamp in northern France. It is due to arrive there on Friday, the data showed.
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.
A 1991 ceasefire was meant to lead to a U.N.-backed referendum on the region's self-determination including the question of independence. But the vote has never happened, and Morocco since 2006 has promoted its own autonomy proposal. (Reporting by Patrick Markey in Algiers and Jonathan Saul in London; Editing by Mark Trevelyan)