(Updates death toll)
SANAA, May 20 (Reuters) - At least 16 people were killed north of the Yemeni capital Sanaa on Tuesday in fighting between Shi'ite Muslim tribesmen and government forces and allied Sunni Muslim rivals, local government sources said.
Bouts of intense sectarian conflict since last year have undermined attempts at national reconciliation in Yemen, a neighbour of major oil exporter Saudi Arabia and home to one of al Qaeda's most active wings.
The sources said the fighting erupted when fighters loyal to the Shi'ite Houthi tribe attacked military and security outposts near the city of Omran, in the province that carries the same name, killing six soldiers and one officer.
The army responded to the attack, killing nine of the fighters.
But a Houthi source said the army opened fire on civilian supporters of the group holding a sit-in and that civilians were killed in the attack.
Violence last erupted there on Saturday when a group of armed Houthi fighters marched to the provincial capital and demanded to stage a demonstration, but were refused entry by the army. Three people were killed.
Fighters loyal to the Houthis, who have repeatedly fought government forces since 2004, are trying to tighten their grip on the north as Yemen eyes moves towards a federal system that will devolve more power to regions.
Earlier this month at least 40 people were killed in clashes between Houthis and tribesmen near Sanaa.
In the southern province of Shabwa, where the army has been waging an offensive against al Qaeda, three militants were arrested, according to state news agency Saba.
The air force also bombed several areas where al Qaeda militants were present, a military source told the agency.
Around 21,000 people have been displaced as a result of the government offensive against al Qaeda in various regions of the impoverished and disordered Arabian Peninsula state.
Gulf Arab states and the United States are concerned about rising violence in the Western-allied country. Yemen has been in turmoil since a popular uprising in 2011 ousted president Ali Abdullah Saleh, and is also struggling with southern secessionists and a protracted economic crisis. (Reporting by Mohamed Ghobari in Sanaa and Mohammed Ghobari in Aden; Writing by Sylvia Westall and Maha El Dahan; Editing by Sami Aboudi and Alison Williams)