The state-run SPA news agency reported Thursday that concern of violent spillover had prompted the alert.
"Anticipating (that) the terrorist organizations or others might carry out actions that might disturb the security of the homeland, the Custodian of the Two Holy Mosques has ordered taking all necessary measures to protect the gains of the homeland and its territories in addition to the security and stability of the Saudi people."
A Saudi official confirmed to CNN the spillover threat posed by the Islamic State of Iraq and Syria.
"Saudi Arabia shares a long border with Iraq and the government is aware that ISIS is very close to Iraq's border with Jordan, and is also aware ISIS has been very public about its intention to attempt to attack Saudi Arabia."
Saudi security forces, the official said, "are at the ready and very strong ..."
But Iraq, he noted, is not the only country of concern; "the threat emanating from Yemen is still very real." The official, who spoke on condition of anonymity, noted that members of al-Qaida in the Arabian Peninsula (AQAP) tried in the last week to surreptitiously enter Saudi Arabia from Yemen.