Video report by ITV News Correspondent John Ray
Boris Johnson returned from holiday and broke his silence on the rising tension in the Middle East.
The prime minister said he wouldn't "lament" the death of Iran's top general Qassem Soleimani - killed in Iraq by a US airstrike - but he did call for a 'de-escalation' of the crisis.
His statement comes amid mass protests in Iran demanding retribution and the Iraqi Parliament calling for the expulsion of all US troops.
Following criticism for remaining on holiday in the Caribbean and failing to issue a statement as tensions escalated, Mr Johnson accused Gen Soleimani of having been “a threat to all our interests”.
"Today I have spoken with President Macron, President Trump and Chancellor Merkel, and will be speaking with other leaders in the coming days," Mr Johnson said.
"General Qassem Soleimani posed a threat to all our interests and was responsible for a pattern of disruptive, destabilising behaviour in the region.
"Given the leading role he has played in actions that have led to the deaths of thousands of innocent civilians and western personnel, we will not lament his death.
"It is clear however that all calls for retaliation or reprisals will simply lead to more violence in the region and they are in no-one's interest.
"We are in close contact with all sides to encourage de-escalation.
"I will be speaking to other leaders and our Iraqi friends to support peace and stability."
Iraq's parliament passed a resolution calling for the expulsion of US troops from the country on Sunday.
The Iraqi parliament held an emergency session where it passed a resolution that says the government must work on ending all foreign troop presence in the country, and asks for the cancellation of a request for assistance from the US-led coalition.
The vote is not legally binding and a final decision will be made by the Government.
The UK Government has urged Iraq to allow British soldiers to remain and continue the fight against so-called Islamic State.
A Government spokesperson said: "The coalition is in Iraq to help protect Iraqis and others from the threat from Daesh (Islamic State), at the request of the Iraqi government.
"We urge the Iraqi government to ensure the coalition is able to continue our vital work countering this shared threat."
US troops were instrumental in Iraq in the fight against so-called Islamic State.
There are currently around 5,200 US troops across Iraq.
Should foreign forces withdraw from Iraq, there are fears it could lead to the resurgence of the terror organisation.
Shortly after the vote, it was announced on Iranian state television that the country will no longer abide by any of the limits set by the landmark 2015 nuclear deal.
On Sunday night, residents of Baghdad say three explosions have been heard inside the heavily-fortified Green Zone, home to the US Embassy and the seat of Iraq's government.
Alert sirens were sounded Sunday in the area on the west bank of the Tigris river.
There was no immediate confirmation from authorities but the explosions were believed to have been from mortars or rockets that struck the area.
Also on Sunday, the US-led military coalition in Iraq announced it is putting the fight against so-called Islamic State on hold to focus on protecting its troops and bases.
The coalition said it is suspending the training of Iraqi forces and other operations in support of the fight against the militants.
General Qassem Soleimani, the head of Iran’s elite Quds Force and mastermind of Iran’s military operations outside of the country, was killed on Friday by a drone strike.
The death of Gen Soleimani marks a major escalation in the standoff between Washington and Iran, which has careened from one crisis to another since President Trump withdrew from the 2015 nuclear deal and imposed crippling sanctions.
The deal was made between the US, UK, Iran, France, Germany and a number of other countries to stop Iran from developing nuclear weapons.
After the US pulled out of the deal, Iran began enriching uranium - a component of nuclear weapons - but has now announced it will not abide by any of the terms set out in the agreement.
The leader of Lebanon's Hezbollah group said America's military in the Middle East region, including US bases, warships and soldiers are fair targets following the killing.
Hassan Nasrallah said evicting US military forces from the region is now a priority.
Hezbollah, a Shiite militant group, is Iran’s key proxy and most successful military export.
Supporters of the group told ITV News their Iranian commander must be avenged and America must "not be allowed to rest, even if it means our children are martyred".
Meanwhile, a former leader of Iran's Revolutionary Guard, of which the Quds force is a part of, said the Israeli cities of Tel Aviv and Haifa can be targeted to avenge Soleimani's death.
Mohsen Rezaee was speaking at a ceremony in Tehran in honour of the general.
He has previously alleged Israel somehow leaked information about Soleimani's whereabouts to US forces.
Israel - which enjoys strong ties with the US - and Iran are longtime foes.
On Saturday night, Donald Trump threatened to strike 52 Iranian sites "very fast and very hard" if Tehran launches an attack on American forces or assets in retaliation.
Iran also accused President Trump of breaching international law in authorising the fatal drone strike, which has created an escalating crisis and sparked fears of all-out war.
Iran’s foreign minister Mohammad Javad Zarif also struck back at the President’s Twitter threat to target 52 Iranian sites “very fast and very hard” if Tehran strikes US assets.
Mr Zarif accused Mr Trump of having “committed grave breaches” of international law with the killing and of threatening to commit a “war crime” by targeting cultural sites.
“Whether kicking or screaming, end of US malign presence in West Asia has begun,” Mr Zarif tweeted.
Outgoing Labour leader Jeremy Corbyn said Mr Johnson "should have immediately cut short his holiday to deal with an issue that could have grave consequences for the UK and the world.”
Emily Thornberry also accused the PM of dismissing her concerns that Mr Trump was heading down a dangerous path by working to tear up the nuclear treaty with Iran.
The Foreign Office issued strengthened travel advice to Britons across the Middle East including Saudi Arabia and Turkey, while the Navy was to begin accompanying UK-flagged ships through the key oil route of the Strait of Hormuz.
Meanwhile, military chiefs were understood to have ordered 400 soldiers training local forces in Iraq to scrap their duties to switch to “force protection” to defend themselves and British diplomats from revenge strikes.
Also on Sunday, General Soleimani's body body arrived back in Iran, where his coffin was carried by tens of thousands of mourners.