"We're totally prepared" for an Iranian attack and to carry out retribution in response to any such action by Iran, Trump told reporters Tuesday afternoon in the Oval Office during a meeting with Greek Prime Minister Kyriakos Mitsotakis.
"They're going to be suffering the consequences and very strongly" if there is an attack, Trump added.
Minutes earlier, at the Pentagon, Defense Secretary Mark Esper had told reporters: "The United States is not seeking a war with Iran, but we are prepared to finish one."
Esper said the U.S. military had increased its force protection posture in the Middle East and is repositioning and bolstering forces in the region.
Esper and other administration officials justified the targeted killing by drone at Baghdad airport of Iranian Major General Qassem Soleimani to prevent further attacks against Americans.
Such an operation, planned by Soleimani, was poised to occur within days, Esper told reporters.
Secretary of State Mike Pompeo told reporters that "multiple pieces of information" from intelligence sources were given to Trump before the president decided to target Soleimani.
Trump stated that Soleimani was in Iraq on "bad business."
National Security Adviser Robert O'Brien said Soleimani was plotting to attack American facilities where he would have killed American "diplomats, soldiers, sailors, airmen and marines."
O'Brien said the case against Soleimani was based on "strong evidence and strong intelligence," but said he could not reveal the "sorts of methods at this time, but I can tell you it was very strong."
U.S. officials are emphasizing Soleimani's years of orchestrating terrorist attacks in the Middle East they contend were not abating.
"If you're looking for imminence, you need look no further than the days that led up to the strike that was taken against Soleimani," Pompeo said, including the late December attack that killed an American contractor working in Iraq.
Trump described the Iranian military leader as "a monster. He's no longer a monster. He's dead."
Iran has vowed to exact revenge for Soleimani's killing.
"We should expect that they will retaliate in some way, shape or form," noted Esper.
O'Brien said Iran's threats are being taken seriously. "We hope that they're deterred, and that they think twice about attacking America and its interests."
Trump, in his Oval Office remarks, appeared to back away from his threats to include Iranian cultural sites in any retaliation.
"We are, according to various laws, supposed to be very careful with their cultural heritage. And you know what, if that's what the law is, I like to obey the law," said Trump. "But think of it: They kill our people. They blow up our people and then we have to be very gentle with their cultural institutions. But I'm OK with it. It's OK with me."
Stampede of mourners
In Iran, officials delayed Soleimani's burial after more than 50 people were killed in a stampede of mourners and more than 200 others injured, according to state media.
Tens of thousands of people had gathered to honor Soleimani in his hometown of Kerman before his planned burial, following similar ceremonies this week in Tehran, Qom and Ahvaz.
Many of the mourners screamed for retaliation against the United States for the killing of Soleimani. "No compromise, no submission, revenge!" they shouted.
Iranian Foreign Minister Mohammad Javad Zarif contended in a CNN interview that the killing of Soleimani, commander of the Revolutionary Guard's Quds Force, constituted "state terrorism."
"This is an act of aggression against Iran, and it amounts to an armed attack against Iran, and we will respond," Zarif said. "But we will respond proportionately — not disproportionately ... we are not lawless like President Trump."
With heightened tensions between the United States and Iran, the State Department has denied Zarif a visa to travel to New York for upcoming United Nations meetings. Pompeo on Tuesday declined to specify the reasons behind the denial of the visa.