U.S. Secretary of State Hillary Clinton says an alleged Iranian-backed plot to assassinate the Saudi ambassador to the U.S. is a “dangerous escalation” in Iran'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, White House spokesman Jay Carney said the plot involved top members of Iran's Revolutionary Guard's Quds Force, an elite military unit that carries out operations outside of Iran. He called that finding “significant in and of itself.”

Carney also said the U.S. is committed to working diplomatically with other countries and the United Nations to isolate Iran, and that Washington is taking “no options off the table” in its response to the alleged plot.

Meanwhile, the U.S. has designated Iranian commercial airline Mahan Air a supporter of terrorist-related activities in Tehran. The designation prohibits U.S. citizens from engaging in commercial or financial transactions with Mahan Air and freezes assets it may hold under U.S. jurisdiction.

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

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 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.