A Boeing 737 crashed shortly after taking off from an airport in Iran early Wednesday, killing all 176 people on board.
Most of the passengers were from Iran or Canada, officials said.
Ukraine International Airlines Flight 752 to Kyiv had left Tehran's Imam Khomeini airport at around 6:10 a.m. local time.
Ukraine's embassy in Tehran initially blamed engine failure for the crash but later updated its statement to say a cause could not be determined until an official inquiry had been carried out.
State media in Iran said the plane crashed due to technical problems. According to Reuters, Iran has said it will not give the plane's black box to Boeing. Ukraine's Foreign Minister Vadym Prystaiko said he had spoken to his Iranian counterpart, Javad Zarif, and agreed to coordinate investigations into the crash.
In a statement, Ukrainian President Volodymyr Zelensky said a team of Ukrainian experts was flying to Iran to take part in the investigation into the crash.
"Our priority is to establish the truth and those responsible for this terrible catastrophe," he said.
Here's the "black box" from the Ukrainian plane that crashed near Tehran earlier this morning. (Iranian state TV)
The plane crash took place just hours after Iran launched a series of ballistic missiles against US and coalition forces at air bases in Iraq. There was no immediate information on whether the crash and the missile attack were linked.
The Federal Aviation Administration in the US banned commercial flights from the region on Wednesday in response to the missile attacks.
The increased military activity and political tensions in the region would put civilian aircrafts at risk if they were misidentified, the FAA notice to aviators said.
Ukraine's Prystaiko said 82 Iranians and 63 Canadians were on board the plane when it came down, as well as 11 Ukrainians including 9 crew members, 10 Swedes, 4 Afghans, 3 Britons, and 3 Germans.
Наразі нам відома наступна інформація щодо країн походження загиблих в катастрофі #PS752: Іран - 82; Канада - 63; Україна - 2 + 9(екіпаж); Швеція - 10; Афганістан - 4; ФРН - 3; Велика Британія - 3. Висловлюємо наші співчуття. Українська влада продовжує розслідування.
“The airline expresses its deepest condolences to the families of the victims of the air crash and will do everything possible to support the relatives of the victims," Ukraine International Airlines (UIA) said in a statement. "With immediate effect, UIA has decided to suspend its flights to Tehran until further notice.”
Ukraine International Airlines confirms air accident in Iran We express our deepest condolences to the families of the victims of the air crash and will do everything possible to support the relatives of the victims. Find more info here: https://t.co/gP3Ow4yWAq
According to the Associated Press, Iranian emergency official Pir Hossein Kulivand said all crew and passengers died in the fiery crash and that authorities were trying to recover the bodies.
The AP also reported that Qassem Biniaz, a spokesperson for Iran's Road and Transportation Ministry, told state-run news agency IRNA that the plane had crashed after a fire appeared to strike one of its engines after takeoff and the pilot lost control.
The jet crashed near Parand, a city outside the airport, according to flight radar.
We are following reports that a Ukrainian 737-800 has crashed shortly after takeoff from Tehran. #PS752 departed Tehran at 02:42UTC. Last ADS-B data received at 02:44UTC. https://t.co/qXWHUPGDTu
Boeing wrote on Twitter, "We are aware of the media reports out of Iran and we are gathering more information."