President Donald Trump has 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" after the strike at Baghdad airport in Iraq 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 the two counties.
US officials have said 3,000 additional troops will be sent to the Middle East as a precaution.
Iraqi state television says 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 have been killed in the fresh strike, which hit a convey of Iraqi militia in the early hours of Saturday morning (local time).
What did President Trump say?
Speaking at a news conference at his Mar-a-Lago resort in Florida, Mr Trump said of Friday's attack: "The United States military executed a flawless precision strike that killed the number one terrorist anywhere in the world Qassem Soleimani."
He said: "Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel but we caught him in the act and terminated him."
How has Iran and Iraq reacted?
In a statement following Soleimani's death, Supreme Leader Ayatollah Ali Khamenei said: "His departure to God does not end his path or his mission, but a forceful revenge awaits the criminals who have his blood and the blood of the other martyrs last night on their hands."
Iraq's parliament will hold an emergency meeting on Sunday.
Prime Minister Adel Abdul Mahdi labelled the missile strike as a "brazen violation of Iraq's sovereignty and a blatant attack on the nation's dignity".
Iraqi militia leader Abu Mahdi al-Muhandis was also killed in the strike. He commanded the Iranian-backed Kataib Hezbollah group, blamed by Washington for a rocket attack that killed a US civilian contractor in northern Iraq last Friday.
The US State Department has warned Americans in Iraq to leave "immediately".
Who was Qasem Soleimani?
The 62-year-old was widely seen as the second most powerful figure in Iran, behind Supreme Leader Ayatollah Ali Khamenei.