Downing Street said it was looking into "very concerning" reports about the Tehran airliner crash as the US claimed the jet was shot down by a missile.
Two US officials said it was "highly likely" an Iranian anti-aircraft missile brought down the passenger plane.
Boris Johnson used a phone call with Ukrainian President Volodymyr Zelenskyy to call for a "full, credible and transparent investigation" into what happened.
Ukraine's Secretary of the National Security and Defence Council, Oleksiy Danilov, posted on Facebook that a missile attack was one of the possible causes of the fire that caused Ukraine International Airlines' (UIA) the PS752 by aircraft to break-up as it gained height after taking off from Tehran airport on Wednesday.
All 167 passengers on board were killed, including three Britons.
Asked whether there were any concerns about the cause of the disaster, a Number 10 spokesman said: "I'm not going to speculate on this but the reports we have seen are very concerning and we are urgently looking into them."
The Iranian military had disputed suggestions the airliner was brought down by a missile, with officials in Iran blaming an engine fire.
But Mr Danilov said "information about the detection of the shards of a Russian missile" – understood to be a Russian Tor M1 missile – meant an attack on the plane could not be ruled out.
In his social media post, he said any investigation would have to also probe whether the plane had been subject to a terrorist attack by someone on-board the flight, or whether it collided with another object or an engine fire occurred for "technical reasons".
The British victims included engineer Sam Zokaei, from Surrey, Saeed Tahmasebi Khademasadi, from west London, and Mohammad Reza Kadkhoda Zadeh, from near Brighton.
All three were named on the list of 167 passengers on board flight PS752 by Ukraine International Airlines (UIA), which crashed moments after it left Imam Khomeini International Airport in Tehran at 6.10am local time (2.40am GMT) on Wednesday, bound for the Ukrainian capital Kyiv.
The Prime Minister pushed for the facts to be established during his conversation with Mr Zelenskyy on Thursday.
Mr Zelenskyy ordered a criminal investigation soon after the details of the tragedy emerged this week.
He has invited United Kingdom aviation experts to join the investigation into the crash.
A spokesman for the President said: "The President invited the United Kingdom to join the investigation.
"Boris Johnson supported this idea and stressed that the best British experts should be involved in clarifying all the circumstances of the tragedy."
The airline ruled out human error in the aftermath of the incident, and crew were not said to have made an emergency call.
The crash comes amid escalating tension in the Persian Gulf following the killing last week of Iranian general Qassem Soleimani by the US, prompting several airlines to reroute flights away from Tehran's airspace.
Iran fired missiles at army bases in Iraq were both US and UK troops were stationed, although both Number 10 and the White House said there had been no casualties incurred.
A Downing Street spokesman said: "He offered his condolences to the president for the loss of the Ukraine International Airlines plane and for all those who were on board.
"President Zelenskyy updated the Prime Minister on Ukrainian efforts to establish the facts and the Prime Minister offered UK support.
"The Prime Minister said that there needed to be a full, credible and transparent investigation into what happened."
During his conversation Mr Johnson also "underlined the UK's continued commitment to Ukrainian sovereignty" with tension over territory continuing to bristle between the eastern European country and Russia.