U.S. Secretary of State Hillary Clinton has called an alleged Iranian-backed plot to assassinate Saudi ambassador to the U.S. a “dangerous escalation” in Tehran's support of terrorism.

Clinton commented on Wednesday, a day after the U.S. Justice Department announced it had charged Manssor Arbabsiar, a naturalized U.S. citizen, and Gholam Shakuri, a member of an elite Iranian military unit, with conspiring to carry out a bomb attack on Saudi envoy Adel al-Jubeir.

U.S. Attorney General Eric Holder said the plot was “conceived, sponsored and directed” from Iran — a charge the Iranian government denies.

On Wednesday, the U.S. announced it had designated Iranian commercial airline Mahan Air as a supporter of terrorist-related activities in Tehran.

A Treasury Department statement says the airline has close ties to Iran Revolutionary Guard's Quds Force, an elite military unit that carries out operations outside of Iran. The statement says the airline was used “secretly' ferry “operatives, weapons and funds.”

The designation prohibits U.S. citizens from engaging in commercial or financial transactions with the airline and freezes any assets that it may hold that are under U.S. jurisdiction.

Meanwhile, U.S. Vice President Joe Biden called the alleged assassination attempt an “outrageous act” for which Iran has to be held accountable.

He told the U.S. television network ABC on Wednesday that “nothing has been taken off the table” regarding the U.S. response to the alleged plot to kill the Saudi envoy.

The U.S. State Department has issued a worldwide alert for American citizens about possible “anti-U.S. actions” following charges against the two Iranians. It said the alleged assassination attempt may indicate Iran's government could be taking a “more aggressive focus” on terrorist activity.

U.S. President Barack Obama called Ambassador al-Jubeir to express solidarity, calling the plot a “flagrant violation” of U.S. and international law.

U.S. officials say Arbabsiar unknowingly hired an informant of the U.S. Drug Enforcement Administration to carry out the plot, believing the informant had ties to Mexican drug cartels capable of killing the Saudi ambassador. They say Arbabsiar confessed to making a $100,000 down payment on the scheme with a price tag of $1.5 million.

Officials arrested Arbabsiar at New York's John F. Kennedy International Airport on September 29, but Shakuri is still at large.

Iran has expressed “outrage” about the the U.S. allegations.