Iran's supreme leader Ayatollah Ali Khamenei vowed "severe revenge" after the killing of the General Qassem Soleimani, the chief of Iran's elite Quds Force in an US airstrike at the Baghdad airport.
Khamenei also declared three days of mourning in the country, reports India Today.
"Martyrdom was the reward for his ceaseless efforts in all these years," Khamenei said on his Twitter account in reference to Soleimani's death.
"With him gone, God willing, his work and his path will not be stopped, but severe revenge awaits the criminals who bloodied their foul hands with his blood and other martyrs' in last night's incident."
"All enemies should know that the jihad of resistance will continue with a doubled motivation, and a definite victory awaits the fighters in the holy war," Khamenei said in a statement telecast by the state TV.
Iran's foreign minister also slammed the killing of Qassem Soleimani and called it a dangerous escalation. The minister also warned that the United States would bear responsibility for the consequences.
"The US' act of international terrorism, targeting and assassinating General Soleimani... is extremely dangerous and a foolish escalation," Mohammad Javad Zarif wrote on Twitter.
"The US bears responsibility for all consequences of its rogue adventurism," he added.
The killing of powerful Iranian military leader Qassem Soleimani is all set to become another flashpoint between the Middle East nation and the US. The move is bound to escalate tensions as US continues to back the move to kill Soleimani while Iran plots revenge.
Top Iraqi militia commander Abu Mahdi al-Muhandis, an adviser to Soleimani, was also killed in the attack, a militia spokesman said.
Iran has been locked in a long conflict with the United States that escalated sharply last week with an attack on the US embassy in Iraq by pro-Iranian militiamen following a US air raid on the Kataib Hezbollah militia, founded by Muhandis.
"At the direction of the President, the US military has taken decisive defensive action to protect US personnel abroad by killing Qassem Soleimani, the Pentagon said in a statement.
"This strike was aimed at deterring future Iranian attack plans," it added.