"A formal designation and its consequences may be new, but these IRGC butchers have been terrorists for a long time", Sen.
The IRGC has always been implicated in terrorist plots against the USA military throughout the Middle East and provides major financial support to other known terrorist organizations such as Lebanese Hezbollah. USA military officials are justifiably concerned about putting more American troops in danger and igniting a conflict with Iran that could undermine the anti-Islamic State campaign in Iraq and Syria as well as Washington's diplomatic relationships in the region.
After US President Donald Trump took office, he withdrew from the Iran nuclear deal and set about clamping down on Tehran's influence in the Middle East, re-imposing a strict sanctions regime that was designed in part to curb the influence of the Guards.
"The Administration's action will increase the financial pressure and isolation of Iran and deprive the regime of resources it uses for its terrorist activities", the White House said.
"We will answer any action taken against this force with a reciprocal action", a statement issued by 255 out of the 290 Iranian lawmakers said, according to IRNA.
The announcement comes shortly before the first anniversary of Trump's decision to pull out of a 2015 nuclear deal with Iran and to reimpose sanctions that had crippled Iran's economy.
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The president's thirst for an agreement with China has grown since the stock market began sliding from its October peak. The administration complains that China has repeatedly failed to keep promises it made in previous trade talks .
Tehran responded in kind, announcing a couple of hours later that its top security council has designated the United States a "state sponsor of terrorism" and US forces in the region "terrorist groups".
The decision came after Foreign Minister Mohammad Javad Zarif urged President Hassan Rohani to place CENTCOM on Tehran's list of "terrorist" groups.
"A (nother) misguided election-eve gift to Netanyahu".
In a statement, Foreign Ministry spokesman Bahram Qassemi said Zarif's request was based on the law to deal with US human rights violations and provocative and terrorist acts in the region, ISNA reported.
A statement signed by a majority of lawmakers in support of the bill said any action against Iran's national security and its armed forces was "crossing a red line" and the U.S. administration would "regret" its decision. Tehran denies the charges.
Iranian Foreign Minister Mohammad Javad Zarif has tweeted his own warning to President Trump: "Netanyahu Firsters who have long agitated for FTO (Foreign Terrorist Organization) designation of the IRGC fully understand its consequences for United States forces in the region".
At home, the Guards have amassed sweeping political and economic influence, with Brian Hook, the State Department´s representative on Iran, citing estimates that they control up to half of the Iranian economy. Monday's designation includes the IRGC's zealous Quds Force, led by the elusive commander Qasem Soleimani and considered Iran's most potent foreign paramilitary organization. Tehran has warned that it has missiles with a range of up to 2,000 km (1,242 miles), putting Israel and United States military bases in the region within reach.