U.S. Vice President Joe Biden says an alleged Iranian-backed attempt to assassinate the Saudi ambassador to the United States is an “outrageous act” for which Iran has to be held accountable. Tehran has denied any involvement in the case.

Biden 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 Saudi ambassador Adel al-Jubeir.

The vice president also said U.S. officials are in the process of, in his words, “uniting world opinion” against Iran.

On Tuesday, the U.S. Justice Department charged Manssor Arbabsiar, a naturalized U.S. citizen with an Iranian passport, and Gholam Shakuri, an Iran-based member of the Iranian Quds force, with conspiring to carry out a bomb attack on al-Jubeir.

U.S. Attorney General Eric Holder said the plot was “conceived, sponsored and directed” from Iran.

The State Department later 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.

Iran's U.N. ambassador wrote a letter to the United Nations Tuesday expressing “outrage” about the the U.S. allegations.

President Barack Obama called Ambassador al-Jubeir to express solidarity against the plot, calling it a “flagrant violation” of U.S. and international law. He assured al-Jubeir the U.S. is committed to protecting all diplomats in the United States.

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 Arabsiar 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.