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

"The US 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 Jan 11, 2012, it said.

US authorities said earlier on Tuesday 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.

Iran denied the charges and expressed outrage at the accusations.

But President Barack Obama called the plot a "flagrant violation of US and international law" and Saudi Arabia said it was "despicable".

The United States said Tehran must be held to account and Secretary of State Hillary Clinton, in a Reuters interview, expressed hope that countries that have hesitated to enforce existing sanctions on Iran would now "go the extra mile".

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, could have been straight from a Hollywood movie.