President Donald Trump said the US killed Iran's top military commander Qasem Soleimani "to stop a war, not to start one".
He said Soleimani's "reign of terror is over" following the strike at Iraq's Baghdad airport on Friday.
Soleimani spearheaded Iran's Middle East operations as head of the Quds Force. Iran has vowed "severe revenge" on those responsible for his death.
The killing marks a major escalation in tensions between Iran and the US.
US officials said 3,000 additional troops will be sent to the Middle East as a precaution.
Meanwhile, Iraqi state television said there has been another air strike in the country, 24 hours after the killing of Soleimani. However, there has been no comment on this from Washington.
An Iraqi army source told Reuters news agency that six people were killed in the fresh strike, which hit a convoy of Iraqi militia in the early hours of Saturday morning (local time).