Western Sahara says Morocco should not be allowed to return to the African Union unless it withdraws from the disputed territory.
Morocco applied to rejoin the AU in July, after pulling out in 1984, when the body recognised Western Sahara's independence.
King Mohammed VI of Morocco has toured various African countries in recent weeks seeking support for the bid.
Moroccan troops went into Western Sahara after Spain withdrew in 1975.
It says Western Sahara is part of its historic territory.
Western Sahara Foreign Minister Mohamed Salek Ould Salek told the BBC that Morocco's occupation contradicted the founding principles of the AU.
"The aim and essence of the Union is the liberation of Africa. Morocco [has] become a coloniser," he said.
"We fought against colonialism and apartheid; how come we are going to accept a country who declared publicly and solemnly that he is occupying a part of a country who is a member of the African Union?" Mr Salek asked.
He insists that if Morocco "wants to join the club, [it] must respect the rules of the club".
Morocco's application will be discussed at the AU summit in January.
