Up to 21 people were killed and hundreds of others injured in ethnic clashes between Berber and Arab communities in Algeria' southern province of Ghardaia, sources told Xinhua news agency on Wednesday.The clashes erupted when a group of unknown youngsters with knives and firearms attacked a private bus. The overnight clashes continued until dawn, a security source said.Since December 2013, the province of Ghardaia has been hit by a series of violent ethnic clashes between the Chaamba community, of Arab origin, and Mozabite Berbers of the Muslim Ibadi sect, which have claimed more than 35 lives so far.A shaky calm was restored last January after the two sides reached a truce initiated by President Abdelaziz Bouteflika. However, this calm did not last more than six months.