Yemeni security officials and a tribal leader say sectarian clashes between northern rebels and ultraconservative Salafis have killed at least 21 people and injured dozens in a northern Yemeni city after a truce fell apart.

Officials say the fierce clashes took place in the early hours of Monday in villages in the northern Arhab mountains, nearly two weeks after ultraconservative Salafis and Hawthi rebels reached a cease-fire after weeks of fighting that killed hundreds. The officials spoke on condition of anonymity because they were not authorized to brief the media.

Arhab tribesmen accused Hawthis, who follow a branch of Shite Islam, of waging a "terrorist invasion" and attempting to drive the tribesmen out of their areas. Meanwhile Hawthi movement spokesman Mohammed Abdel-Salam accused the government of backing the Salafis.