A confidential U.N. document says Morocco violated a 1991 cease-fire agreement with the Polisario Front independence movement by sending armed security personnel and equipment into the contested Western Sahara region without prior notice to U.N. peacekeepers.

The note to the U.N. Security Council from the Department of Peacekeeping Operations, obtained Monday by The Associated Press, said the Polisario Front deployed 32 armed military personnel in response, also in violation of the cease-fire.

Secretary-General Ban Ki-moon expressed deep concern Sunday at "the tense situation that has developed in the narrow buffer strip in southwestern Western Sahara between the Moroccan berm and the Mauritanian border," his spokesman said.

Ban's spokesman said the U.N. chief urged both sides to respect "the letter and the spirit of the cease-fire agreement."