Ayatollah Ali Khamenei, Iran's Supreme leader, said on Friday the assassination of Quds Force chief Qassem Soleimani would double the motivation of the resistance against the United States and Israel, state television reported.
The veteran army officer was killed by a US airstrike on Friday morning at Baghdad airport, sparking tensions in the Middle East.
"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.
Iran often refers to regional countries and forces opposed to Israel and the United States as a "resistance" front.
The US accused Soleimani of organising deadly protests at its Iraq embassy earlier this week, which led to the death of a number of people.
Soleimani was the head of the deadly Iran Revolutionary Guard Corps, who are responsible to weeding out of ISIS and Al-Qaida.
"The Hashed confirmed that both Soleimani and its deputy chief Abu Mahdi al-Muhandis were killed in what it said was a "US strike," Iran state media said.
"The deputy head of the Hashed, Abu Mahdi al-Muhandis, and head of the Quds Force, Qasem Soleimani, were killed in a US strike that targeted their car on the Baghdad International Airport road," it said.
On Thursday, US Secretary of Defence Mark Esper told reporters there could be more attacks from Iran-backed groups but vowed American forces would respond.
"We are prepared to exercise self-defence, and we are prepared to deter further bad behaviour from these groups, all of which are sponsored, directed and resourced by Iran," he said