Video report by ITV News Washington Correspondent Robert Moore
Both Boris Johnson and Canadian Prime Minister Justin Trudeau have suggested Iran shot down a passenger plane near Tehran which crashed and killed all 176 people on board.
Four of the passengers - the number was revised up from the earlier figure of three - on board the Ukraine airliner on Wednesday were British, while 63 were Canadian and 82 were Iranian.
The Prime Ministers' statements come after two US officials said it was "highly likely" Iran was responsible for the plane crash but it "may well have been unintentional".
The crash comes amid the backdrop of heightened tensions between Iran and the US, after the killing of Iranian General Qassem Soleimani sparked missile attacks from Tehran against US airbases in Iraq.
The Ukrainian passenger plane crashed several hours after Iran launched attacks on two US military bases in Iraq, and journalists asked Mr Trudeau if the aircraft may have been accidentally caught in the crossfire.
It is though the plane was brought down by accident, with US satellites spotting that two missiles were launched at the time of the incident.
Speaking at a press conference in Ottawa, Mr Trudeau said: "What happened yesterday was a tragedy which shocked not only Canada, but the world."
"We have intelligence from multiple sources including from our allies.
"This information suggests that the plane was shot down by an Iranian surface to air missile.
"This may well have been unintentional.
"This new information reinforces the need for a further investigation into this matter.
"Canada is working with its allies to ensure that a thorough and credible investigation is conducted to determine that causes of the crash."
Mr Trudeau told those gathered he wanted "answers" and that his government would "not rest" until it is established exactly what happened.
Mr Trudeau was also asked if Mr Trump was in part responsible for what happened, since he sparked the current rise in tensions by ordering Gen Soleimani's killing, however, the Canadian PM refused to be drawn on this.
Shortly after Mr Trudeau spoke, Mr Johnson issued his own statement which echoed that of the Canadian PM's.
“The loss of life on Ukrainian International Airlines Flight 752 is a tragedy and my thoughts are with all those who lost loved ones," Mr Johnson said.
“Four British nationals were among those who were killed, and we are providing support to their families at this most terrible time.
“There is now a body of information that the flight was shot down by an Iranian Surface to Air Missile.
"This may well have been unintentional.
“We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation.
“It is vital that there should be an immediate and respectful repatriation of those who’ve lost their lives to allow their families to grieve properly.
“The UK continues to call on all sides urgently to deescalate to reduce tensions in the region.”
Also on Thursday, US President Donald Trump said he believes "somebody could have made a mistake" to bring the plane down.
"Well I have my suspicions, I don't want to say that because other people have those suspicions also.
"It's a tragic thing... somebody could have made a mistake on the other side, it was flying at a pretty rough neighbourhood and somebody could have made a mistake
"Some people think it might be mechanical, I don't think its mechanical personally."
He added: "I have a feeling that something very terrible happened, very devastating."
The comments come after Iranian investigators said the Ukrainian plane did not make a radio call for help and was trying to turn back to the airport when it went down, with officials instead saying the crash was caused by an engine fire.
Iran’s civil aviation authority also said that the plane was engulfed in flames before it crashed.
The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the flight to the Ukrainian capital, Kyiv, the report said.
Iran also confirmed that both of the black boxes that contain data and cockpit communications from the plane had been recovered, though they had been damaged and some parts of their memory was lost.
Iran said they would not hand over the black boxes to the US.
It also invited Ukraine and Boeing to take part in the investigation, despite earlier saying it would not hand the black boxes over to the American owned plane manufacturer.
Meanwhile, Ukrainian President Zelenskiy said crash investigators from his country had arrived in Iran to assist in the probe.
He also said he planned to call President Hassan Rouhani about the crash and the investigation.
“Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash,” Mr Zelenskiy said.
“We will surely find out the truth.”
Aviation experts have been skeptical about Iran's initial claim the plane was brought down by a mechanical problem.
Mr Zelenskiy laid flowers at Kyiv Airport on Thursday at a memorial for the victims of the crash.
Three of the four Britons who died in the crash have been named as Sam Zokaei from Surrey, Saeed Tahmasebi Khademasadi from west London and Mohammad Reza Kadkhoda Zadeh from near Brighton.
Canadian Foreign Minister François-Philippe Champagne spoke to his Iranian counterpart Mohamed Javad Zaraf on Thursday.
Mr Champagne expressed the importance of allowing Canadian officials into the country to help with consular matters, identify the deceased and aid the investigation into the crash.