Riyadh: A Saudi border guard was killed Thursday along Yemen's frontier, raising to nine the death toll for the kingdom's armed forces since a Saudi-led air war on Yemeni rebels, the interior ministry said.

The air strikes launched on March 25 to halt an advance by Iran-backed Al Houthi rebels have sparked exchanges of fire along the border region, where all the Saudi casualties have occurred.

The latest Saudi victim was the driver of a border guards vehicle delivering supplies in Jizan district, the interior ministry said in a statement carried by the official Saudi Press Agency.

The patrol was hit by a "military projectile", the statement said without elaborating.

Saudi Arabia has reinforced the frontier with artillery, tanks and hilltop lookout posts to block any Houthi incursion from their traditional highland stronghold just over the border.

The UN says about half of the more than 1,000 people killed in the Yemen fighting since late March were civilians, and millions of people have been affected by the war.

The Houthis are allied with army units loyal to former Yemeni president Ali Abdullah Saleh.

They are fighting loyalists of President Abedrabbo Mansour Hadi, who fled the rebels' southern advance for Riyadh in March.