RIYADH (AFP) - More than 500 Yemeni Shi'ite rebels have died in clashes with the Saudi military on the border since Riyadh launched an air war in late March, the defence ministry said Saturday.

It was the first reported death toll for border clashes since Saudi Arabia, heading an Arab military coalition, launched air strikes against Shiite Huthi rebels in Yemen on March 26.

On the Saudi side, the death toll doubled on Friday when three soldiers were killed and two wounded as mortar fire hit their observation post in Najran province, a ministry spokesman said, quoted by state news agency SPA.

Saudi authorities say three border guards were killed last week in an attack by the Huthis, who control parts of northern Yemen along the border.