Video report by ITV News Washington Correspondent Robert Moore
The withdrawal of US troops from Iraq would be the "worst thing", Donald Trump has said.
The US President's words came on the day Iraq's outgoing Prime Minister said American forces must pull out.
Speaking to reporters alongside Greece's PM in a meeting which was dominated by the US's assassination of Iran's top general, Mr Trump said the killing of Qassem Soleimani saved a lot of lives because he was planning a "very big" and "very bad" attack.
He continued that the operation had been based on what he called tremendous information about his plans to attack US interests.
Mr Trump and his top advisers are under pressure to disclose more detail about the intelligence that led to the killing, which has greatly heightened tension with Tehran.
“We saved a lot of lives,” he said.
“They were planning something.”
Mr Trump said: “He’s been called a monster, and he was a monster, and he’s no longer a monster, he’s dead.”
Secretary of State Mike Pompeo said it was clear that Gen Soleimani was continuing his efforts to build a network of activities “that were going to lead potentially to the death of many more Americans”.
While Defence Secretary Mark Esper told reporters that Iranian threats against Americans were “’days away” from being executed.
However, when pressed on Iran's plans National Security Advisor Robert O'Brien became tongue-tied and said the country had "been making threats publicly everyday".
However, Democratic lawmakers are not convinced.
Senate Minority Leader Chuck Schumer and Senator Bob Menendez, the top Democrat on the Senate foreign relations committee, called on Mr Trump to declassify the written notification the president sent to Congress after the fatal strike on Gen Soleimani.
Speaking on Tuesday, the US President went on to warn Iran will "suffer the consequences very strongly” if it does “anything thing they shouldn’t be doing”, such as launching a retaliation.
He added the US is “prepared to attack” Iran, something Mr Esper said should be expected.
President Trump had previously threatened to target cultural sites in Iran – which would violate international law – but said on Tuesday he “likes to obey the law”, suggesting this would not happen.
However, he continued: “They're [Iran] allowed to kill our people, they're allowed to maim our people, they're allowed to blow up everything that we have,” he told reports in the White House.
“And 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 - l like to obey the law - but think about it they kill our people they blow up our people then we have to be very gentle with their cultural institutions but I'm OK with it.”
He later added: “If Iran does anything they shouldn't be doing they're going to be suffering the consequences and very strongly.”
The President also claimed he knew nothing of a draft letter from a senior military officer that appeared to suggest plans for the withdrawal of US troops were under way.
He said he did not know if the letter – reportedly a mistake that should not have been sent – was a “hoax” and asked Mr Pompeo “was it unsigned or what?”
Earlier on Tuesday, a stampede at the funeral procession for Gen Soleimanileft at least 56 people dead and 213 injured, state television reported.
It is estimated up to a million people took to the streets to pay tribute to the former Quds commander in his hometown of Kerman in Iran.
Mourners dressed in black and carried posters bearing the image of Soleimani, a man whose slaying prompted Iran’s supreme leader to weep over his casket.
There is no information as to what set off the stampede.
Meanwhile Foreign Secretary Dominic Raab travelled to Brussels to "de-escalate" the situation with his European counterparts, as he said the only winners of any potential war with Iran would be so-called Islamic State.
Speaking to reporters from RAF Northolt, he said: "What we are looking to do is to de-escalate the tensions with Iran.
"We are concerned that if we see a full-blown war, it would be very damaging and actually the terrorists, in particular Daesh (ISIS) would be the only winners."
Mr Raab spoke on Monday to his Iranian counterpart, Mohammed Javad Zarif, to stress the need for a diplomatic resolution to avoid a renewed conflict in the region.
The Foreign Office confirmed to ITV News that Britain's ambassador to Tehran was called in to the foreign ministry there on Tuesday and warned that Britain must not become America's partner in this crime.
ITV News Political Correspondent Carl Dinnen on where the UK stands in the escalating tensions and the warning to Britain's ambassador
Meanwhile, ITV News obtained a letter from the Prime Minister to Jeremy Corbyn, turning down his request for an urgent Privy Council briefing about the assassination of Gen Soleimani.
Political Editor Robert Peston said that Boris Johnson's refusal to bring Corbyn into his confidence raises concerns not only about the legality of the assassination ordered by Mr Trump, but also whether our security services believe the US President had sound reason to see Soleimani as a clear and present threat to American personnel.
Rather than the PM update MPs on the latest situation in the Middle East on the first day back after the Christmas break, it was instead Defence Secretary Ben Wallace who told the Commons the government has taken "urgent measures" to protect British personnel in Iran, including putting helicopters and ships in the region on standby.
"General Soleimani was no friend or our allies in the region, he was not an advocate of a more peaceful or prosperous Middle East," Mr Wallace said.
"Further conflict is in no one's interest, the only beneficiary would be the terrorists and extremists seeking to use the chaos as cover to advance their abhorrent objectives."