Iran's Supreme National Council has moved swiftly to brand the United States as a "state sponsor of terrorism", in a tit-for-tat response to President Donald Trump's designation of the Islamic Republic's elite Revolutionary Guard Corps (IRGC) as a "foreign terrorist organisation".
In a statement carried on Monday by Iran's state-run IRNA news agency, the council said it also declared all US forces operating in the Middle East "terrorist groups".
The developments mark a new high point in the already tense relations between Tehran and Washington, which took a turn for the worse in the wake of the US withdrawal last year from a nuclear deal brokered with Iran and other world powers in 2015.
Iranian Foreign Minister Javad Zarif, for his part, criticised Trump's decision as "another dangerous US misadventure" in the Middle East and a "misguided election-eve gift" to Benjamin Netanyahu, the prime minister of Israel, Iran's regional archenemy.
Netanyahu, an ally of Trump, thanked the US president and said it would help in "keeping the world safe from Iran aggression and terrorism".
Trump vows to keep up pressure
The IRGC was set up to protect the country's Shia clerical ruling system after the 1979 Islamic Revolution, which toppled the Western-allied secular monarch Shah Mohammad Reza Pahlavi and led to the formation of the Islamic Republic headed by Ayatollah Ruhollah Khomeini.
With an estimated 125,000 personnel, comprised of army, navy and air units, the force is Iran's most powerful security organisation.
It controls the country's ballistic missiles and nuclear programmes, answers directly to Supreme Leader Ayatollah Ali Khamenei and owns a vast network of businesses ranging from oil and gas projects to construction and telecommunication.
Trump said in a statement on Monday that the designation "recognises 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".
His move marked an unprecedented step by the US against an entire institution of a foreign government.
Trump said the decision "underscores the fact that Iran's actions are fundamentally different from those of other governments" and vowed to continue increasing financial pressure on Tehran until "it abandons its malign and outlaw behaviour".
"It makes crystal clear the risks of conducting business with, or providing support to, the IRGC. If you are doing business with the IRGC, you will be bankrolling terrorism," he said.
Trump's administration has already put sanctions in place on more than 970 Iranian individuals and entities, according to the US State Department.
US Secretary of State Mike Pompeo, a strident critic of Iran, confirmed the new designation would take effect one week from Monday.