The US has imposed sanctions on an Iranian airline it says flew members of an elite force linked to an alleged plot to kill the Saudi envoy to the US, BBC reported.
The US Treasury says Mahan Air ferried operatives from Iran's Quds Force and Hezbollah across the Middle East.
Under the sanctions, the airline's US assets will be frozen and US firms barred from doing business with it.
Secretary of State Hillary Clinton meanwhile said the alleged plot was a "dangerous escalation" by Iran.
The sanctions were unveiled a day after the authorities announced they had foiled a conspiracy to assassinate the Saudi ambassador to Washington, Adel Al-Jubeir, on US soil using explosives.
'Secretly ferrying operatives'
The US government accused members of the Iranian government - and the Quds Force, an elite unit of the Revolutionary Guard Corps - of involvement.
"It's clear that senior levels of Quds force were engaged in the plotting," White House press secretary Jay Carney told reporters.
On Tuesday, the US also imposed sanctions against five people it linked to the alleged assassination plot, including two men charged over the investigation.
The pair were named as Manssor Arbabsiar, a 56-year-old naturalised US citizen with dual Iranian and US passports, and Gholam Shakuri, who is based in Iran.
Mr Shakuri and three others named in Tuesday's sanctions were described as members of the Quds Force.
Mr Arbabsiar - accused of wiring $100,000 (£63,000) to a US bank account to finance the alleged $1.5m conspiracy - was charged in a New York City court on Tuesday.
US officials have said that the accused approached a US informant posing as a member of a Mexican drug cartel to carry out the assassination.
'Mischievous scenario'
Announcing the latest sanctions on Wednesday, Treasury official David Cohen said in a statement: "Mahan Air's close co-ordination with the IRGC-QF [Islamic Revolutionary Guards Corp-Quds Force] - secretly ferrying operatives, weapons and funds on its flights - reveals yet another facet of the IRGC's extensive infiltration of Iran's commercial sector to facilitate its support for terrorism."
Iranian official media reported on Wednesday that the foreign ministry had summoned the Swiss ambassador to Tehran, who represents US interests in the country.
The purpose of the meeting was to "strongly" protest against the US allegations, the state television website reported.
Iranian Foreign Minister Ali Akbar Salehi was quoted by Iran's Arabic-language Alalam channel as saying: "America has launched a mischievous scenario. But be certain, they will apologise [to Iran] in the future."
The Gulf Cooperation Council condemned the alleged plot as a "flagrant violation" of international laws and agreements.
Meanwhile, details have emerged of Mr Arbabsiar's life in the US, including his background as a secondhand-car salesman in Texas.
David Tomscha, who once owned a used car lot with him, told AP news agency: "I can't imagine him thinking up a plan like that. I mean, he didn't seem all that political. He was more of a businessman."
Mr Arbabsiar was charged with theft in 2001, according to court documents in Texas, although the charge was dropped. He was also arrested several times in the 1990s for traffic violations.
His wife, Martha Guerrero, who lives at a house in an Austin suburb, told a local TV station they were separated and she thought he was blameless of the charges.
"I cannot for the life of me think that he would be capable of doing that," she told KVUE. "I'm sure of that and I know that his innocence is going to come out."
The case has strained already fraught relations between Washington and Tehran.
Iranian Supreme Leader Ayatollah Ali Khamenei told a mass rally on Wednesday that the Occupy Wall Street protests in the US would topple the American capitalist system.