DUBAI (Reuters) - Yemeni Shi'ite Muslim rebels said they had seized mountainous territory inside Saudi Arabia, and Saudi authorities said on Wednesday a security officer had been killed by gunmen crossing from Yemen.

Saudi Arabia said the attack took place on Tuesday morning in a border region next to the northern part of Yemen, where Yemeni government forces have been battling the rebels.

"Complete control has been taken yesterday evening of Jabal al-Dukhan ...after attacks during which the aggressor was eliminated," the rebels said in statement posted on their website late on Tuesday.

The rebels have accused Saudi Arabia of involvement in their conflict with the central government in Sanaa.

Western countries and Saudi Arabia fear that al Qaeda will gain momentum from an insurrection by Shi'ite rebels in north Yemen and separatist sentiment in the south.

(Reporting by Raissa Kasolowsky and Andrew Hammond)