The United States has ordered nonessential diplomats out of Saudi Arabia and warned all Americans they should leave.
The warning cited fresh signals that attacks are planned on US and Western interests.
The decision requires the families of all diplomats at the US Embassy in Riyadh and consulates in Jeddah and Dhahran to leave the kingdom, which is battling violence believed to be linked to al-Qaeda.
On Tuesday suspected Muslim militants killed four Saudi police officers at checkpoints soon after security forces defused two car bombs in the capital. At least 50 people died last year in suicide bomb attacks on housing compounds in Riyadh.
"The US government has received recent and credible information indicating that extremists are planning further attacks against US and Western interests," the State Department said in a travel warning.
"We are concerned. The threat level has gone up," Secretary of State Colin Powell said.
Washington has repeatedly moved to reduce its diplomatic presence in Saudi Arabia, the world's biggest oil exporter, and then allowed diplomats and family members back in response to the ebb and flow of threats over the past year.
In February it announced it had allowed all workers and family members to return to the kingdom. Yesterday, the State Department repeated past warnings that residential compounds in Riyadh and the rest of the country continue to be targeted.