Unidentified gunmen have killed two Saudi border guards who were patrolling Saudi Arabia’s border with Yemen, the Saudi state news agency said on Thursday.

The gunmen opened fire on a border patrol early on Wednesday in Dhahran al-Janoub in the southwestern province of Asir, SPA reported, but did not say who was behind the attack.

Saudi Arabia, wary of traffickers and militants trespassing from Yemen, has been building a security fence along its 1,800 km (1,100 mile) border with the impoverished country since 2003.