U.S. President Donald Trump announced Monday that the U.S. is designating Iran's Revolutionary Guard a "foreign terrorist organization," in an effort to increase pressure on the country that could have significant diplomatic implications in the Middle East. Iran's Supreme Council quickly responded by designating U.S. forces as terrorist organization.

Iran's state-run TV said on Monday that Washington's decision to designate Iran's Islamic Revolutionary Guard Corps a foreign terrorist organization contradicted international law and was illegal.

Iran's Foreign Minister Zarif tweeted that Trump's designation is just "A(nother) misguided election-eve gift to Netanyahu. A(nother) dangerous U.S. misadventure in the region."

"No other country has the legal right to designate as terrorist another country's armed forces ... Iran's influence in the Middle East and its success in fighting against Islamic State are reasons behind this designation," state TV said, without quoting a specific official.

Trump's move is the first time that the U.S. has designated a part of another government as a terrorist organization.

The designation imposes sanctions that include freezes on assets the Islamic Revolutionary Guard Corps may have in U.S. jurisdictions and a ban on Americans doing business with it.

"This unprecedented step, led by the Department of State, recognizes the reality that Iran is not only a State Sponsor of Terrorism, but that the IRGC actively participates in, finances, and promotes terrorism as a tool of statecraft," Trump said in a statement.

Iran has threatened to retaliate for the decision.

The IRGC is a paramilitary organization formed in the wake of Iran's 1979 Islamic Revolution to defend its clerically overseen government. The force answers only to Iran's supreme leader, operates independently of the regular military and has vast economic interests across the country.