SANAA: Security officials say more than 40 people have been killed in sectarian clashes between militants and northern rebel forces in northern Yemen.

The officials say the fighting began when Salafis took over a Houthi stronghold in a strategic mountainous area near the border with Saudi Arabia. The two sides battled with artillery fire, mortar shells and machine guns in the town of Al-Fagga.

Thursday’s clashes mark an expansion of the battlefield between the two rival sides, which have been concentrated in an adjacent province.

Meanwhile, the UN closed its offices in the Yemeni capital on Thursday over fears of possible car bomb attacks, although Western embassies remained open.

Despite official denials of an alert, most shops in Sanaa were shuttered and little traffic ventured out onto the streets as rumors swirled among the city’s residents of the risk of an imminent attack. The American and Turkish schools were also closed.