Donald Trump has said the killing of Iran's top military chief, General Qassem Soleimani, was to "stop a war".
He said: "We took action last night to stop a war. We did not take action to start a war."
The US president said the Iranian military chief General Qassem Soleimani had been plotting "imminent and sinister attacks" on American diplomats and military personnel.
He added: "We caught him in the act and terminated him."
Mr Trump said the US was not seeking regime change in Iran, but claimed the world was a safer place without "monsters" such as Gen Soleimani.
He said: "If Americans anywhere are threatened we have all of those targets fully identified and I am ready and prepared to take whatever action is necessary, and that particularly refers to Iran."
It comes as Iran has threatened retaliation after Soleimani - tipped as a future leader - and members of his entourage .
The airstrike has been condemned by Iraq, which said the attack could "spark a devastating war in Iraq".
In a press conference, Trump said the recent attacks on US targets in Iraq and the assault on the US embassy in Baghdad were at Soleimani's direction.
Trump said the US acted to stop a war, not to start one and his "most solemn duty is the defense of the nation".
He said he had deep respect for the Iranian people and was not seeking a regime change.
However, Trump said he is ready and prepared to take whatever action is necessary to protect Americans.
Soleimani was reportedly "torn to shreds" in the attack on his convoy.
Two senior Islamic Revolutionary Guard figures, Brigadier General Hussein Jafari Nia and Major-General Hadi Taremi also died in the blasts.
Earlier today it was announced that 3,000 extra troops would be deployed to the Middle East following the assasination.
US military officials, speaking on condition of anonymity, told the troops would be joining the roughly 750 forces that were sent to Kuwait earlier this week.
US officials told Reuters earlier this week that thousands of additional troops could be sent to the region and had been told to prepare to deploy.
