White House Spokesman Jay Carney said Iran's Quds Force was behind the alleged Iranian-backed plot to kill the Saudi ambassador to Washington.
On Wednesday U.S. federal prosecutors in New York have charged two Iranians with plotting to assassinate the Saudi ambassador to the U.S., Adel Al-Jubeir.
"It's clear that senior levels of Quds Force were engaged in the plotting," he said, adding that the U.S. will continue to deepen Iran's isolation with fresh sanctions.
"We're responding very concretely with actions we know will have an impact on Iran and will make clear this kind of behavior is unacceptable and will further isolate Iran," Carney said.
US Attorney-General Eric Holder said on Wednesday the plot was masterminded, sponsored and directed from Iran, but declined to disclose details of the conspiracy and confirm that it was approved and ordered directly by Iranian authorities.
Iran almost immediately dismissed the accusation. A spokesman for Iranian leader Mahmoud Ahmadinejad said the plot was "prefabricated" and aimed at distracting the society from domestic problems.
Saudi Arabia strongly condemned the assassination attempt and said it would take "resolute actions" to prevent further attempts to undermine the kingdom's security. The statement didn't mention Iran, however.
"The government of Saudi Arabia appreciates the efforts by U.S. to foil this conspiracy," the country's SPA news agency quoted a government statement.
"The Kingdom will take resolute actions to stop those criminal activities and repel any attempt to destabilize [the country] and threaten its security," the statement reads.
The Russian Foreign Ministry also expressed its concern about the foiled assassination plot, saying it would study the available information "in the most thorough way possible."
"We are convinced that effective counter-action against terrorist threats lies in boosting international cooperation," the foreign ministry said in a statement. "To achieve this goal, we are ready to cooperate with all states concerned, including the United States."
EU foreign policy chief Catherine Ashton said the plot, if succeeded, "would constitute a major breach of international law with serious international implications."
