At least 13 Shiite Houthi fighters and four Sunni tribal militiamen were killed Monday during clashes between the two sides, on the borders between the Yemeni provinces of Marib and Al Bayda, the Spanish international news agency (EFE) reported. A tribal source, who asked to remain anonymous, told EFE that Sunni tribal fighters in the province of Marib were engaging in fierce battle to defend their land against Houthi fighters who are seeking to seize control of the area. Yemen has witnessed violent clashes between the Houthi fighters and the national forces causing instability and chaos in the country. The Houthis participated in the Yemeni demonstrations of 2011 that ousted Yemeni President Ali Abdullah Saleh. They took part in the National Dialogue Conference in February 2014, which called for making Yemen a federation of six regions.