The Prime Minister has remained silent over the US's fatal strike on Iran's top general throughout his trip to the private island of Mustique to celebrate the New Year with his partner Carrie Symonds.
Mr Johnson is under mounting pressure from opposition leaders to make a statement on the killing of General Qassem Soleimani and rising fears of all-out war after Iran threatened revenge over the Donald Trump-approved attack in Baghdad on Friday.
"Everyone will take revenge," the president responded.
Mr Trump later spoke of the increase in US military spending and tweeted: "They attacked us, & we hit back. If they attack again, which I would strongly advise them not to do, we will hit them harder than they have ever been hit before!"
Boris Johnson has yet to make any public statement on the killing of General Soleimani, however today Foreign Secretary Dominic Raab said Soleimani was a "regional menace" and the US had "the right of self defence".
He added: "If you look at what General Soleimani was doing, he's not some victim in this scenario. He was a regional menace he was in charge of the Quds Force, the wing of the Iranian Revolutionary Guard which is directing militias and proxies in the region, in Iraq, in Lebanon, in Syria which is destabilising those countries, trying to get an Iranian advantage and trying to attacking western countries that are legitimately there.
"So we understand the position the US were in and I don't think we should be naive about the Iranian Revolutionary Guard or indeed General Soleimani."
Mr Raab also defended the Prime Minister's absence and said he had been in "constant contact" with Boris Johnson.
Asked whether Mr Johnson should return from break on Sky's Ridge On Sunday, Mr Raab said: "The Prime Minister is in charge. In fact I've been in constant contact with him over the Christmas break on a whole range of foreign policy issues.
"We were in touch on Friday with relation to the situation in Iraq and the whole Government is working very closely together. I spoke to the (US) Defence Secretary last night, I talked to the (US) National Security Adviser on Friday and we're very clear on the strategy and how we're implementing it and he'll be back in play tomorrow in the UK."
"I've been hitting the phones hard in relation to all our international partners, and as you said, I'll be travelling and meeting with our European partners, our American friends, I'll be in Montreal meeting my Canadian opposite number as well.
"And so the diplomatic effort goes on and indeed the Prime Minister has been engaged in that as well."
Mr Raab also confirmed that he has a meeting with US Secretary of State Mike Pompeo next week.
He said the UK is looking to deescalate the tensions between Iran and the US in the Middle East, adding that there needs to be "route through this which allows Iran to come in from the international cold".
Mr Raab refused to say if UK troops in the Middle East are in more danger, but admitted that there are "heightened tensions".
He said: "People can rest assure that we are doing everything we can from the travel advice, through protecting our military and diplomatic missions, to keep people as safe as possible."
Mr Raab was also asked about the two British-Iranian dual citizens currently in jail in Iran who are concerned over their fate amid the increasing tensions according to their relatives/
Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori are serving five and 10 year sentences respectively in Tehran's notorious Evin Prison on charges of spying.
Mr Raab said that the issue was "at the forefront of my mind" and that hardliners in Tehran "have not complied with their obligations under international law" and have "not been responsible members of the international community".
"We have seen intransigence from the regime in relation to all of our dual nationals, I think they've been subject to appalling treatment, as the dual nationals of other countries have, so that is part of the nefarious behaviour we've seen from the hardliners in Tehran."
Mr Raab said he had spoken to the Iraqi prime minister and president and will be speaking to Iran's foreign minister. He warned a war was in "no one's interests" and would only benefit the so-called Islamic State as he stressed the need to "avoid" a slide into accidental war and "de-escalate the situation".
"I spoke to the Iraqi prime minister just this morning, the Iraqi president last night and I will be reaching out to the foreign minister of Iran with that same message."
The Foreign Office has issued strengthened travel advice to Britons across the Middle East including Saudi Arabia and Turkey, and the Navy will begin accompanying UK-flagged ships through the key oil route of the Strait of Hormuz.
Former Labour foreign secretary Jack Straw said he has "real concern" about the implications of the US assassination of General Soleimani.
Speaking to BBC News on Sunday, Mr Straw said: "Even if you accept, which I don't, President Trump's assertions that Qassem Soleimani was, quote: 'the world's worst terrorist leader', this was not a sensible action to take and it will have really serious implications for the West, for America and for the region as a whole.
"The other thing I thought when I heard about this, was how petrified the Iranians would then be, because of the penetration of their security screen around Qassem Soleimani.
"He is the second-most well-protected individual in the Iranian regime, and the Iranians will be neurotic to an extreme now, that their security and secrecy surrounding his movements was penetrated."
However Mr Straw agreed with Foreign Secretary Dominic Raab's calls for a de-escalation of tensions in the region. He said: "I think the Government has got it right up to now. If they (Iran) are angry about the fact they were completely blindsided by the US administration over this operation, I don't blame them at all. It's absolutely extraordinary.
"Now we have President Trump talking about 52 targets in Iran... I am not clear whether he has really worked out where this could end up."
Shadow foreign secretary also Emily Thornberry warned of a "lurch towards war" and said Mr Trump's decision had been "reckless".
"To take him out at this stage when there has been escalating tensions seems to me to be not making the world safer, actually we are taking a major lurch towards war. And we are doing that because the president is reckless and hasn't thought through what it is he is doing.
"But it seems to me quite clear that the Iranians are going to counter-attack and it means that our interests, our people, our forces are, of course, under threat."
Ms Thornberry also criticised Mr Trump for failing to notify the UK before carrying out the fatal drone strike. "He didn't even tell us before he agreed for this man to be killed in Baghdad.
"They breached Iraqi sovereignty in order to kill the head of the defence forces for Iran. There will be great pressure on the Iraqis to say to the Americans and to the British and to the other Western allies that have been trying to help them fight Isis that they should leave, and so I think that it makes the whole area much more unstable, much more dangerous."
Ms Thornberry continued her criticism of Mr Johnson's lack of responsibility while on holiday in the Caribbean. "We should take responsibility, we are international players, of course, we have other preoccupations, and clearly the Prime Minister has a lot of preoccupations - he's sunning himself drinking vodka martinis somewhere else and not paying attention to this.
"We've had three Cobra meetings where Mark Sedwill, the chief civil servant, has had to chair it because the Prime Minister hasn't been available."