TUNIS — At least 18 people have been killed in clashes between Berbers and Arabs in several desert towns in Algeria this week, the Algerian news media reported on Wednesday. Several buildings were burned and one police officer was wounded in clashes on Monday and Tuesday. Longstanding disputes between the native Berbers and Arab settlers over land and housing in the eastern province of Ghardaia turned violent two years ago, and the government has struggled to contain the continuing unrest. The latest casualties occurred even after the arrival of police reinforcements and a visit by Interior Minister Nourreddine Bedoui to the town of Ghardaia a week ago.