Albania's Constitutional Court says it has annulled an agreement with neighboring Greece marking out maritime borders and the continental shelf in the Ionian Sea.

The agreement was found unconstitutional "due to procedural and substantial violations," a court statement said Wednesday. The full decision was not immediately available.

Six Albanian parties, including the main opposition Socialists, had challenged the April 2009 agreement, accusing the governing Democrats of lack of transparency in negotiations as well as gifting sea area to Athens.

Foreign Minister Ilir Meta said the government would respect the ruling.

Relations with Greece have been strained in the past, mostly over the treatment of more than 1 million Albanian immigrants in Greece and of an ethnic Greek minority in Albania.