Saudi Arabia vowed yesterday that Iran would "pay the price" for an alleged plot to assassinate the Saudi ambassador to the United States that has further destabilised the already fraught relationship between the Middle East's two premier powers.
After the US made the extraordinary allegations, it took steps yesterday to increase global pressure on the Tehran regime. These included sanctions against an Iranian airline and fresh moves at the United Nations.
In London, the former Saudi intelligence chief Prince Turki al-Faisal said the evidence against Iran was "overwhelming - and clearly shows official Iranian responsibility for this. Somebody in Iran will have to pay the price."
There were also unsubstantiated allegations from the US and Saudi Arabia that responsibility for the plot went to the very top.
As US officials speculated that it was "more than likely that the Supreme Leader Ayatollah Ali Khamenei" knew of the plot, the Saudi-backed news network Al-Arabiya quoted official sources as claiming that President Mahmoud Ahmadinejad was the "mastermind" behind it and had recently suggested Iran "resume its assassination policy".
Iran's Foreign Minister, Ali-Akbar Salehi, said the best reaction to the claims would be to "ignore" them.
Sunni Saudi Arabia and Shia Iran have long been at odds, and tensions have risen during the Arab Spring with allegations that Iran bolstered a Shia uprising against the Saudi-backed Sunni regime in Bahrain.
As reactions oscillated between outrage and bafflement yesterday, the State Department briefed the diplomatic corps about the plot.
Simultaneously, US ambassadors abroad were urging their host governments to take steps to punish Iran and further isolate it on the world stage.
In New York, Susan Rice, the US ambassador to the UN, was meeting other members of the Security Council.
It was being stressed yesterday that there was never any danger to Adel al-Jubeir, the envoy and close adviser of King Abdullah, who was supposed to be killed - along with dozens of Americans too if necessary - while dining at a restaurant.
The Treasury Department imposed sanctions on Mahan Air, an Iranian airline which it said provided funds and transport for Iran's security forces. Hillary Clinton, the Secretary of State, accused Tehran of "undermining the international system" by violating conventions protecting diplomats.
Policymakers and analysts were asking why Tehran would have embarked upon an action extreme which might have provoked a devastating military retaliation from the US.
Some said the recruitment of Manssor Arbabsiar, a car salesman with dual Iranian and US citizenship who was to enlist hired hitmen from Mexican drug gangs, made no sense as a strategy for the Quds Force, which is usually depicted as a highly efficient and sophisticated organisation.