A Boeing 737-800 NG carrying 176 people on board crashed in Iran shortly after take-off, reportedly killing all on-board.
The plane, operated by Ukrainian International Airlines, took off from Imam Khomeini international airport in Tehran at 6.12am bound for Kiev in Ukraine.
The jet, which was less than four years old and is reported to have had its last scheduled maintenance check on Monday, came down around eight minutes after taking off.
Ukraine has said that 'engine failure' caused the crash and initially said that there was no link to terrorism, but speculation around the cause of the crash remains.
Ukraine's foreign ministry website said: "According to preliminary information, the plane crashed as a result of an engine failure for technical reasons. Currently, the version of a terrorist attack is ruled out."
There were 168 passengers and nine crew on board the flight, according to the Ukranian prime minister.
Among the victims were 82 Iranians, 63 Canadians, 11 Ukrainians including all the crew, 10 Swedes, four Afghans, three Britons and three Germans, Ukrain'es foreign minister Vadym Prystaiko said.
Iran's Red Crescent told state media that it was "impossible" that anyone could have survived the crash.
A video circulating on social media and among news agencies claims to show the aircraft burning as it falls from the sky. The video is yet to be authenticated.
Boeing issued a statement saying: "@This is a tragic event and our heartfelt thoughts are with the crew, passengers, and their families. We are in contact with our airline customer and stand by them in this difficult time. We are ready to assist in any way needed."
Ukraine's president Volodymyr Zelensky said in a satement: "My sincere condolences to the relatives and friends of all passengers and crew."
Ukraine International Airlines has suspended all flights to Tehran indefinitely.
This story is being updated.