Video report by ITV Washington Correspondent Robert Moore
Almost 3,000 US troops will be sent to the Middle East after an American rocket killed an Iranian military chief, Washington has said.
It comes after Iran has warned of "harsh retaliation" following a US airstrike that killed Tehran's top general, Qasem Soleimani.
On Friday leaders of both Iran and the United States fired rhetoric at one another, with an adviser to Tehran stating: "Trump, through his gamble, has dragged the US into the most dangerous situation in the region."
The airstrike was quickly followed by a chorus of appeals for a climbdown in tensions between Iran and the United States.
Governments around the world have been vying to find a consistent position on the killing that deescalates tensions.
In Iranian cities mourners protested against the death and the US intervention, a match strike against the tension tinder box that is the Middle East.
Video report by ITV News Security Correspondent Rohit Kachroo
US President Donald Trump - who had earlier in the night tweeted an image of an American flag - said the general was responsible for many deaths abroad and in Iran.
He said that he "should have been taken out many years ago".
Why did Trump order the killing of Soleimani?
In a speech from Mar-a-Lago in Florida, the president said the US "acted to stop a war, it did not take action to start a war."
"Under my leadership, American's policy is unambiguous to terrorists, who harm or intend to harm any American, we will find you, we will eliminate you, we will always protect our diplomats, service members, all Americans and our allies.
"We took action last night to stop a war, we did not take action to start a war."
Mr Trump said the Iranian general was plotting "imminent and sinister attacks" and added: "We caught him in the act and terminated him."
Defence officials, who spoke on condition of anonymity to discuss a decision not yet announced by the Pentagon, said the troops are from the 82nd Airborne Division at Fort Bragg, North Carolina.
The United States urged American citizens to leave Iraq "immediately".
U.S. embassies have issued a security alert for Americans in Lebanon, Bahrain Kuwait and Nigeria.
Around 5,200 American troops are already based in Iraq to train Iraqi forces and help in the fight against Islamic State group militants - that number will almost double with this new deployment.