Iran told the United States on Tuesday that it will keep "all options on table" if President Donald Trump designate its elite Revolutionary Guards (IRGC) as a terrorist organization.
It came a day after the government said Washington itself would be aiding terrorism if it took such an action.
US President Donald Trump is expected to announce this week his final decision on how he wants to contain Iran's regional influence. He is also expected to designate Iran's most powerful security force, the Islamic Revolutionary Guards Corp as a terrorist organization.
"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.
Iranian government spokesperson also said that the world should be "thankful" to the Revolutionary Guards for its fight 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.
Trump is 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 but give Congress 60 days to decide whether to reimpose sanctions.
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 spokesperson 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.