Iranian Supreme Leader Ayatollah Ali Khamenei has threatened "severe retaliation" against the "criminals" who killed Qasem Soleimani, the powerful commander of Iran's Quds Force who died in a U.S. air strike in western Baghdad on January 3.
Khamenei also declared three days of national mourning to mark Soleimani's death.
Soleimani was killed in a possible drone strike while traveling in a two-car convoy near Baghdad's international airport early on January 3.
The U.S. military said it carried out the "defensive" strike to prevent Soleimani from orchestrating further attacks against U.S. interests in the region.
Khamenei said the U.S. action will "double the motivation" of the Iranian people's resistance against the United States.
Iranian President Hassan Rohani said in televised remarks that the assassination will make Tehran more decisive in its resistance against the United States.
"Soleimani's martyrdom will make Iran more decisive to resist America's expansionism and to defend our Islamic values. With no doubt, Iran and other freedom-seeking countries in the region will take his revenge," Rohani said, according to Reuters.
Iranian media said the Foreign Ministry had summoned the Swiss charge d'affaires, whose country represents U.S. interests in Iran in the absence of formal diplomatic relations between Washington and Iran, and informed the envoy of its "strong protest."
"He was told that Washington's move is a blatant instance of state terrorism and the US regime is responsible for all its consequences," Press TV reported.
Iranian news agency IRNA quoted Defense Minister Amir Hatami as saying Tehran would extract "crushing revenge" for Soleimani's killing, according to Reuters.
U.S. President Donald Trump tweeted later on January 3 that "Iran never won a war, but never lost a negotiation!"
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Trump administration officials have repeatedly urged Tehran to return to the negotiating table after Washington withdrew from the 2015 nuclear deal between Iran and world powers, but Iran's leaders have insisted they won't engage in talks until U.S. sanctions are lifted.
The Quds Force is the elite foreign arm of Iran's Islamic Revolutionary Guards Corps and has been declared a foreign terrorist organization by the United States.