British Prime Minister Boris Johnson called Thursday for a "full, credible and transparent" investigation into how a Ukrainian passenger plane crashed in Iran, killing all 176 people on board.
A spokesman said reports about what happened were "very concerning", after a Ukrainian security official said investigators were pursuing several leads, including the possibility of a surface-to-air missile strike.
Johnson spoke to Ukrainian President Volodymyr Zelensky in a phone call in which he offered his condolences for the loss of life and received an update on Ukraine's efforts to establish the facts, the spokesman said.
"The prime minister said that there needed to be a full, credible and transparent investigation into what happened," the spokesman said.
He added: "The reports we have seen are very concerning and we are urgently looking into them."
The Ukraine International Airlines (UIA) Boeing 737NG went down on Wednesday, shortly after Tehran launched missiles at US forces in Iraq in response to the killing of a top Iranian general in a US drone strike in Baghdad.
Oleksiy Danilov, the secretary of Ukraine's National Security and Defence Council, told AFP that investigators were looking at a number of leads.
These include a collision with another airborne object, a rocket from Iran's missile defence system, an engine explosion caused by a technical problem, and an explosion on board the aircraft due to an "act of terror", he said on Facebook.
Danilov told AFP that for the moment there was no reason to believe that the airliner had been hit by a missile.
In Kiev, the Ukrainian presidency said Zelensky invited Britain to join the probe, and also called for a "transparent, thorough and objective investigation that can quickly establish the facts of this tragedy".
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