At least 13 people died when Shi’ite Muslim fighters clashed with security forces in northern Yemen on Friday, government sources said, underlining the chaos still gripping the country two years after mass protests ousted its leader.

The fighters were from the Houthi movement which is seeking to strengthen its hold on the north – one of a string of challenges facing an interim government also battling southern separatists, al Qaeda-linked militants and an economic crisis.

The security situation in Yemen is closely watched in Gulf Arab states and Washington given the impoverished country’s strategic position next to top oil exporter Saudi Arabia and to main shipping lanes.