The State Department late issued a worldwide travel alert for U.S. citizens, warning of the potential for anti-U.S. action after the United States accused Iran of backing a plot to kill Saudi Arabia's ambassador to Washington.

"The U.S. government assesses that this Iranian-backed plan to assassinate the Saudi ambassador may indicate a more aggressive focus by the Iranian government on terrorist activity against diplomats from certain countries, to include possible attacks in the United States," it said in a statement on its website.

The alert expires January 11, 2012, it said.

U.S. authorities said that they had broken up a plot by two men linked to Iran's security agencies to assassinate Saudi Ambassador Adel al-Jubeir. One was arrested last month while the other was believed to be in Iran.

At a news conference, FBI Director Robert Mueller said the convoluted plot, involving monitored international calls, Mexican drug money and an attempt to blow up the ambassador in a Washington restaurant.