U.S. President Barack Obama and Saudi King Abdullah have pledged to pursue a “strong and unified” international response to the alleged plot by elements of the Iranian government to assassinate the Saudi ambassador to Washington.

The White House said Wednesday that Mr. Obama and the King agreed during a telephone call that the plot represents a “flagrant violation” of international norms and vowed to hold “those responsible accountable for their actions.”

Earlier, U.S. Secretary of State Hillary Clinton called the thwarted scheme a “dangerous escalation” in Iran's “long-standing use of political violence and sponsorship of terrorism.”

Clinton said Wednesday the U.S. will work closely with its international partners “to increase Iran's isolation and the pressure on its government.”

The tough response came 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.

White House spokesman Jay Carney said the plot involved top members of the Iranian Revolutionary Guard's Quds Force, a group that operates outside of Iran. He called that finding “significant in and of itself,” and said Washington is taking “no options off the table” in its response to the alleged scheme.

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

U.S. Vice President Joe Biden called the alleged assassination attempt an “outrageous act” for which Iran must 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 scheme may indicate Iran's government could be taking a “more aggressive focus” on terrorist activity.

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.