Iran told the United States on Tuesday that it will keep "all options on table" if President Trump designates its elite Revolutionary Guards (IRGC) as a terrorist organization.
It came hours after the government said Washington itself would be aiding terrorism if it took such an action.
President Trump is expected to announce this week his final decision on how he wants to contain Iran's regional influence.
Mr. Trump is also expected to "decertify" a landmark 2015 deal Iran struck with world powers to curb its nuclear program in return for the lifting of most international sanctions. Trump's announcement would stop short of pulling out of the agreement, punting that decision to Congress, which would have 60 days to decide whether to reimpose sanctions.
He is also expected to designate Iran's most powerful security force, the Islamic Revolutionary Guards Corps, as a terrorist organization.
US sanctions on the IRGC could affect conflicts in Iraq and Syria, where Tehran and Washington both support warring parties that oppose the Islamic State militant group.
"The Americans are too small to be able to harm the Revolutionary Guards," Ali Akbar Velayati, the top adviser to Iran's Supreme Leader Ayatollah Ali Khamenei was quoted as saying by ISNA. "We have all options on the table. Whatever they do, we will take reciprocal measures," he added.
The Iranian nuclear deal, agreed in 2015 and supported by European countries, Russia, and China, lifted international sanctions on Iran in return for it agreeing to curbs on its nuclear program.
'Firm, decisive and crushing'
Washington maintains separate unilateral sanctions on Iran over its missile program and allegations that it supports terrorism in the Middle East. It already blacklists some individuals and entities for supporting IRGC activities, but not the Guards themselves.
The Guards have a vast economic empire in Iran. Designating them terrorists could make it more difficult for some Iranian businesses to take advantage of the lifting of sanctions to interact with global banks, which are required to verify that their clients are not on terrorism blacklists.
Iran’s rial has dropped against the US dollar in recent days in a sign of concern about Trump’s policy. The rial was quoted in the free market around 40,400 to the dollar, currency exchangers in Tehran told Reuters, compared to 39,200 last week. Several exchangers said they had stopped selling dollars from Monday and were waiting to assess the trend in the market.
An Iranian government spokesman said that the world should be "thankful" to the Revolutionary Guards for fighting against the Islamic State and other terrorist groups.
"By taking a stance against the Revolutionary Guards and designating it a terrorist group, the Americans would be joining the terrorists' camp," Mohammad Baqer Nobakht said in a weekly news conference broadcast live on state television.
IRGC commander Mohammad Ali Jafari said on Sunday that if Washington designated the Guards a terrorist organization, they "will consider the American Army to be like Islamic State all around the world."
Iran’s Foreign Ministry spokesman Bahram Qasemi said on Monday that Tehran would give a "firm, decisive and crushing" response if the United States goes ahead with such a plan.
Washington aims to put more pressure on the IRGC, especially over its missile program. Trump said in September that recent IRGC missile tests illustrated the weakness of the nuclear deal reached by his predecessor, former President Barack Obama.
Iranian Foreign Minister Mohammad Javad Zarif said Iran purposefully excluded its military capability from the nuclear deal, as "it is not intended as leverage or a bargaining chip in future negotiations."
In an article published in the Atlantic on Monday Zarif added: "No party or country need fear our missiles … unless it intends to attack our territory."
This story was reported by Reuters.