Three officers were instantly killed when the outpost came under attack, the Saudi Press Agency reported, without specifying who was behind it.

Local authorities apprehended two suspects and a third opened fire as he tried to escape, killing the fourth officer.

Authorities did not name the suspects.

Saudi Arabia has been embroiled in a long running conflict with southern neighbour Yemen since March 2015.

In March 2015, the kingdom launched a coalition of Arab states fighting to roll back Iran-backed Huthi rebels in Yemen and restore the country's internationally-recognised government to power.

Riyadh and Shiite rival Tehran back opposing sides in a range of hotspots across the mainly Sunni Muslim Middle East, including Lebanon, Syria, Iraq and Yemen.

Last November, Saudi Crown Prince Mohammed bin Salman vowed to "pursue terrorists" as he presided over the first meeting of an Islamic counter-terrorism alliance involving around 40 Muslim countries.