Iran's top security body vowed to retaliate in the "right place and time" after the United States killed Revolutionary Guards commander Qasem Soleimani in Baghdad on Friday.
"America should know that its criminal attack on General Soleimani has been the country's biggest mistake in west Asia, and America will not avoid the consequences of this wrong calculation easily," the Supreme National Security Council said in a statement.
"These criminals will face severe vengeance... in the right place and time," it added after holding an extraordinary meeting following Soleimani's death.
The council described Soleimani as a "glorious general" who was the "pride not only for Iranians but all Muslims and downtrodden... across the world".
But it added that although his death was a "great loss", his role would be taken up by another general.
It also asked whether the "progress of Iran's armed forces and its defensive capabilities has ever been slowed by the martyrdom of its great generals".
Soleimani was killed by a US air strike outside Baghdad international airport on Friday.
Iran's supreme leader Ayatollah Ali Khamenei said later that Soleimani's deputy in the Guards' Quds Force, Esmail Qaani, would replace the slain commander.
In its statement, the Supreme National Security Council said the United States would "be responsible for the consequences of all aspects of this criminal adventure".
It said it believes that "the blood of this great martyr... will have many blessings for the great nation of Iran and other freedom-seekers of the world."