Video report by ITV News Washington Correspondent Robert Moore
Donald Trump said Iran appears to be "standing down" after it launched a series of ballistic missiles at two US airbases.
The US President adopted a conciliatory tone when he provided an update on the damage caused by Iran's airstrikes on two US military bases in Iraq, which were in retaliation to the killing of General Qassem Soleimani.
Flanked by high-ranking members of the US military and his administration, he said: "Iran appears to be standing down, which is a good thing for all parties concerned and a good thing for the world."
He added he would introduce "powerful sanctions " against Iran until it "changed its behaviour" following its airstrikes.
Speaking to reporters at the White House, Mr Trump also confirmed there were no casualties from Iran's late-night bombing.
Trump says US will impose "powerful sanction" against Iran
"No Americans were harmed in last night's attack by the Iranian regime.
"We suffered no causalities, all of our soldiers were safe and only minimal damage was sustained at our military bases," he said.
Despite an apparent de-escalation in tensions, Mr Trump urged the UK, Germany, France, China and Russia to abandon the Iran nuclear deal, which was designed to limit Tehran's ability to develop nuclear weapons but was signed under Barack Obama's administration.
Following the drone strike which killed general Soleimani on Friday, Iran said it would no longer abide by the deal.
Mr Trump claimed Soleimani was planning imminent attacks on US and its allies and that the general's hands were "drenched in the blood of American troops".
Senior International Correspondent John Irvine reports on the missiles launched at the US air bases in Iraq
The US President also urged the Islamic Republic to "abandon its support for terror" and warned he would never let Iran develop nuclear weapons.
"As long as I am President of the United States, Iran will never be able to have a nuclear weapon," he said.
"The very defective JCPOA expires shortly anyway and gives Iran a clear and quick path to nuclear breakout," he told a press conference at the White House.
ITV News Washington Correspondent Robert Moore explains the impact of Trump's statement
"Iran must abandon its nuclear ambitions and end its support for terrorism.
"The time has come for the United Kingdom, Germany, France, Russia and China to recognise this reality.
"They must now break away from the remnants of the Iran deal - or JCPOA - and we must all work together towards making a deal with Iran that makes the world a safer and more peaceful place."
Mr Trump used the platform to highlight the US "did not need" oil from the Middle East.
Trump urges UK to break away from Iran nuclear deal
Boris Johnson said he condemns the "reckless and dangerous" missile attacks on the US Iraqi bases and he urged Iran to "pursue urgent de-escalation."
He spoke with Canadian president Justin Trudeau on Wednesday evening, and the pair discussed "the need for urgent de-escalation on all sides".
A Number 10 spokesperson added: "On the nuclear deal, the leaders committed to continue working together and with international partners to ensure Iran is prevented from acquiring a nuclear weapon."
Speaking at his first PMQs since the General Election, the prime minister said HMS Defender and HMS Montrose were in a "state of readiness" to protecting shipping in the Gulf, especially in the Strait of Hormuz.
Mr Johnson went on to say the legality of the killing of Soleimani was "not for the UK to determine" as it was a US operation.
Footage shows the moments the missiles are launched
But he added: "Soleimani was not only responsible for...arming the Houthi rebels with missiles attacking innocent civilians, arming Hezbollah with missiles..sustaining the Assad regime in Syria, one of the most brutal and barbaric regimes in the world, and supplying improvise explosive devices to terrorists who killed or maimed British troops.
"That man had the blood of British troops on his hands."
Jeremy Corbyn accused the prime minister of "prioritising" his relationship with President Trump as he has "hitched his wagon to a trade deal" with the US, after Brexit.
But Mr Johnson responded, calling the claim "absolute fiction."
Mr Johnson said there were no US casualties and no British personnel were injured in the attacks.
The British government has urged all sides to "de-escalate" tensions.
Brexit secretary Stephen Barclay said a "huge amount of activity" is going on within government to improve relations between Iran and the US, saying "it's in no side's interest to see further tension".
Iranian Supreme Leader Ayatollah Ali Khamenei in an address to the nation says "we slapped them (Americans) on the face last night" with a missile strike but "military action is not enough."
Khamenei added the "corrupt presence of the U.S. in the region should come to an end," saying it has caused war, division, and destruction.
Iran's supreme leader also invoked the virtues of the slain commander, Gen Soleimani, saying he was a "great, brave warrior" and "dear friend to us."
Huge crowds turned out in Iran to mourn Soleimani's death, where he was buried in his hometown of Kerman on Tuesday.
ITV News Washington Correspondent Robert Moore said there are a limited number of British soldiers stationed at Al-Asad, which he described as "the crown jewels of American military facilities".
He said the situation is currently a "very dangerous" one of "escalation".
Mr Trump's response to the attack will "undoubtedly be the most consequential decision for the President since he came to office".
Iran's Revolutionary Guard has also warned the US's regional allies against retaliating against the missile attack.
“We are warning all American allies, who gave their bases to its terrorist army, that any territory that is the starting point of aggressive acts against Iran will be targeted,” the Guard said.
It also threatened Israel.
Just hours after the missile attack, all passengers and crew on board died after a plane crashed shortly after takeoff from Tehran’s main airport, as it travelled to Ukraine.