A U.S. diplomat in Riyadh declined to say if the upgrade was in response to a specific terrorist threat.
The Riyadh Diplomatic Quarter, a residential area covering five square miles west of the city, has been heavily guarded by Saudi armed forces since May 12 suicide attacks on foreigners' housing compounds.
The U.S. Embassy repeated its advice earlier this month that U.S. citizens in the kingdom should "remain vigilant, maintain a low profile and follow prudent security measures."
It also urged Americans to "avoid places and areas where Westerners are known to congregate, including residential compounds."
When the warning was first issued, U.S. diplomats said they were not recommending that U.S. citizens evacuate the housing compounds, where expatriate workers tend to live in Riyadh. But they said American diplomats could only visit the compounds on official business.
Suicide bombers killed 35 people, including the nine assailants, in the May attacks on the compounds. Another 17 people were killed in a second suicide attack on the compounds on Nov. 8.
"The embassy continues to be concerned about the current situation in Saudi Arabia," Saturday's warden message said. "Effective immediately, travel of all embassy personnel and dependents off the Riyadh Diplomatic Quarter should be restricted to essential business only."
"Similar restrictions will be placed on American personnel and dependents at the consulates in Jiddah and Dhahran," the message added.
The U.S. Embassy in Riyadh and the consulates in Jiddah and Dhahran closed on Nov. 8, hours before the suicide attack on the Riyadh compound. The three missions reopened a week later, with the embassy saying that while it would monitor the situation, it was now safe to resume business.
The Australian and British embassies also upgraded their security warnings shortly before Nov. 8, saying they had received information of a specific threat.
Earlier this month, Britain repeated its warning on Saudi Arabia, telling its citizens it advised against "all but essential travel" to the kingdom.
"Following terrorist attacks in Riyadh in May and November, we continue to believe terrorists are planning further attacks in Saudi Arabia," the British Embassy said on Dec. 2.