BRUSSELS (Reuters) - The government of Alberta, home to Canada's huge deposits of oil sands, has written to EU experts voicing "grave concerns" the bloc's plans to rank unconventional oil as a highly polluting fuel are unfair and a potential threat to trade ties.

"The proposed measure has been deliberately crafted in such a way as to discriminate specifically and uniquely against oil sands derived fuels," according to a copy of the letter seen by Reuters.

"Alberta believes that the Fuel Quality Directive implementing measure as it currently stands would be incompatible with the EU's international trade obligations."

The oil sands debate coincides with difficult talks between Canada and the European Union on a proposed free trade deal. The two sides have yet to agree on a range of issues, including intellectual property and access for agricultural goods.

(Reporting by Charlie Dunmore and Barbara Lewis; editing by Rex Merrifield)