WASHINGTON, Oct 12: US Vice President Joe Biden warned Iran on Wednesday that it would have to face ‘serious consequences’ for allegedly plotting to assassinate the Saudi ambassador to the United States. US President Barack Obama called the plot a “flagrant violation” of US and international law but allowed his deputies to do most of the talking.

Also on Wednesday, the US imposed sanctions on an Iranian commercial airline – Mahan Air – for allegedly “ferrying operatives, weapons and funds” on behalf of the Islamic Revolutionary Guard Corps, which is also known as the Quds force.

“The consequences for Iran, I think, will be serious. I think what we have to do is unite the entire world against the Iranian behaviour,” Mr Biden told CBS News.

At the White House, spokesman Jay Carney told reporters that senior members of Iran’s Quds force had participated in the alleged plot to assassinate the Saudi envoy, Adel al-Jubeir.

“It’s clear that senior levels of Quds force were engaged in the plotting,” he said, adding that Washington would respond by intensifying efforts to isolate Iran.

US Secretary of State Hillary Clinton said the alleged plot was a “dangerous escalation” in Iran’s support of terrorism.

The plot is “a flagrant violation of international and US law and a dangerous escalation of the Iranian government’s long-standing use of political violence and sponsorship of terrorism”, she said.

On Tuesday afternoon, the US Justice Department announced that it had charged Manssor Arbabsiar, a naturalised US citizen, and Gholam Shakuri, a member of the Quds force with conspiring to carry out a bomb attack on the Saudi envoy.

In Tehran, Iranian Foreign Minister Ali Akbar Salehi called the plot “an amateurish scenario” and told the news agency ISNA there have been similar allegations over the past few decades.

“The Islamic Republic never seeks to get involved in this kind of behaviour and, despite 32 years of pressure brought to bear on Iran, the country has always acted and reacted ethically,” he said. But the Royal Embassy of Saudi Arabia in Washington endorsed the US claim saying that the plot was “a despicable violation of international norms, standards and conventions” and said it was “not in accord with the principles of humanity”.

In London, Saudi Prince Turki al-Faisal told a conference that “someone in Iran is going to have to pay the price” for the plot.

Vice President Biden said the US was laying out its case to world leaders and demanded “accountability for Iran and further isolation of Iran in terms of their ability to operate around the world”.

Iran was already subject to numerous American sanctions but on Wednesday, the US Treasury Department expanded the sanctions to include Mahan Air, claiming that it was “yet another facet of the Islamic Revolutionary Guard Corps’ extensive infiltration of Iran’s commercial sector to facilitate its support for terrorism”.

On Capitol Hill, Republican lawmakers joined Democrats in condemning Iran.

“This would constitute an act of war not only against the Saudis and Israelis but against the United States,” said Congressman Michael McCaul.

“Iran’s assassination of a foreign diplomat in our country would have violated both US and international law, and represented an act of war,” said Congressman Peter King.

Congressman Ted Poe called the plot an “act of war” against the United States and said: “We have to do something.”

But a senior US Defence Department official told Fox News the announcement was “not a trip wire for military action” in Iran.

“No one should read into this as preteens for any type of military response,” the official said.