At least 34 people have been killed in clashes between Shi'ite rebels and pro-government tribes in north Yemen in the past three days. Officials said in the Harf Sufyan district, at least 28 people died in fighting between Houthi rebels and the government-allied Aziz tribe. In a separate incident late yesterday in Saada, security officials say Houthi rebels ambushed and killed a tribal chief, his son and four others. The Houthis have, however, denied responsibility for the attack. The Yemeni government struck a truce with the Houthis in February, after a six-year conflict. But instability still threatens Yemen and neighboring Saudi Arabia, which was briefly drawn into the conflict last year when rebels seized Saudi border areas.