A day after the US accused Iran of plotting to kill Saudi Arabia's ambassador on its soil, authorities now say they are taking the case to the UN.
A Western diplomat in Riyadh said on Wednesday the charges would likely be discussed at the UN Security Council.
"The US and Saudi Arabia and other allies are discussing the possibility of taking this to the Security Council because this is an assault on a foreign diplomat in the US," he said.
Barack Obama, the US president, spoke to King Abdullah and both "agreed that this plot represents a flagrant violation of fundamental international norms, ethics, and law", according to a statement released by the White House.
Hillary Clinton, the US secretary of state, said Iran's alleged ties to the plot marked a ''dangerous escalation'' of Tehran's sponsorship of terrorism.
"Iran must be held accountable for its actions," Clinton said. Countries hesitant to enforce existing sanctions on Iran would now "go the extra mile", she said.
The secretary of state said Washington was already stepping up sanctions against those in the government associated with the plot.
US to seek more sanctions
The White House said the US would seek further sanctions against Iran.
"We're responding very concretely with actions we know will have an impact on Iran and will make clear this kind of behaviour is unacceptable and will further isolate Iran," Jay Carney, the White House spokesman, told reporters.
Joe Biden, the US vice-president, said Washington was working for a new round of international sanctions against Iran, warning that "nothing has been taken off the table".
Also on Wednesday, the US treasury department slapped sanctions on Iran's Mahan Air, saying it had secretly transported members of the Quds Force, an unit of the Iranian Revolutionary Guards.
The airline's assets in the US will now be frozen and US citizens will be barred from doing business with the firm.
The Gulf Co-operation Council (GCC), which groups Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and United Arab Emirates, joined European nations in condemning the plot and said it was "severely harmful" to Gulf-Iranian relations.
Abdulattif al-Zayani, the secretary-general of GCC, said in a statement he considered the plot a "flagrant violation" of all laws and international agreements.
Iran denies role
Iran has vigorously rejected the US allegations that it had backed a plot to kill the Saudi ambassador to Washington, with a senior official describing them as a "childish, amateur game".
Ali Larijani, the parliament speaker, said that the "fabricated allegations" aimed to divert attention from Arab uprisings that Iran says are inspired by its own Islamic revolution that toppled the Shah in 1979.
"These claims are vulgar. We believe that our neighbours in the region are very well aware that America is using this story to ruin our relationship with Saudi Arabia."
The Persian nation, which faces four rounds of UN sanctions over its nuclear programme, sent a letter of protest to the Security Council accusing Washington of "warmongering".
Brigadier General Hossein Salami, the deputy commander of Iran's Revolutionary Guards, called the allegations "ridiculous and baseless".
Saudi condemns the plot
But Saudi Arabia strongly condemned the alleged plan to kill its ambassador, the SPA state news agency reported, quoting an unnamed official.
"The Kingdom strongly condemns the sinful and abhorrent attempt to assassinate the [Saudi] envoy... to the Unites States," SPA said, quoting the official.
It said Iran would "pay the price" for the alleged plot.
|Mohammad Marandi, an Iranian political analyst, discusses the US accusations
"The burden of proof is overwhelming... and clearly shows official Iranian responsibility for this. Somebody in Iran will have to pay the price," senior Saudi prince Turki al-Faisal, a former ambassador to Washington, said in London.
The primary evidence linking Iran to the alleged conspiracy is that the arrested suspect is said to have told US law-enforcement agents that he had been recruited and directed by men he understood were senior Quds Force officials..
The US authorities have named two suspects: Manssor Arbabsiar, an Iranian but also a naturalised US citizen, and Gholam Shakuri, said to be an Iran-based member of the Quds Force.
Shakuri is believed to be in Iran, while Arbabsiar was arrested on September 29 at New York's John F Kennedy airport and appeared in court on Tuesday in Manhattan. His lawyer said he would plead not guilty, if charged.