A Ukrainian Boeing 737-800 with 176 people on board has crashed in Iran, and officials say there is no chance of finding survivors.
Ukraine International Airlines flight PS752 to Kyiv went down just minutes after taking off from Tehran's airport at 06:12 local time (02:42 GMT).
The majority of passengers were from Iran and Canada.
Ukraine's Tehran embassy initially blamed engine failure but later removed the statement.
It said any comment regarding the cause of Wednesday's accident prior to a commission's inquiry was not official.
There was good visibility when the plane went down near Iran's capital, according to the Flightradar24 aviation website. Officials from the airline said the crew were experienced.
Ukraine International Airlines (UIA) said the Boeing 737-800 was built in 2016 and had its last scheduled maintenance on Monday.
Hours before the plane came down Iran carried out a ballistic missile attack on two air bases housing US forces in Iraq. There is no evidence that the two incidents were linked.
Ukraine's president, Volodymyr Zelensky, warned against "speculation or unchecked theories regarding the catastrophe" until official reports were ready.
What are Iranian authorities saying?
Iranian media blamed technical problems and quoted an aviation official who said no emergency had been declared.
In a sign of the potential difficulties facing crash investigators, the head of Iran's civil aviation organisation was quoted as saying the Ukrainian plane's "black boxes" - the cockpit voice recorder and the flight data recorder - had been found but would not be handed over, either to Boeing or the Americans.
Ali Abedzadeh said "terrorism" had played no role in the crash, Iran's conservative Mehr news agency reported. He added that at this stage it was unclear who would analyse the black boxes.
Another Iranian official, Qasem Biniaz, blamed an engine fire. "Had the accident happened due to a missile strike, the plane would have exploded in the air," he told the IRNA news agency.
Normally, the US National Transportation Safety Board would have a role to play in any international investigations involving US-made Boeings. But the board must act with permission and in accordance with legislation of the foreign country concerned.
Video circulating on social media appeared to show the last few seconds of the plane's flight. It can be seen streaking across the night sky while on fire before it quickly loses height. There is an initial explosion close to the ground and then a final blast that lights up the area.
Debris and engine parts from the Boeing 737-800 NG plane were found some 10km (6 miles) from Tehran's Imam Khomeini airport to the west of the capital, and rescue workers with face masks searched the wreckage for victims.
Throughout the morning, Red Crescent workers laid out a long line of body bags. Personal effects such as children's books, clothes and shoes lay strewn around the area, alongside the mangled debris of the plane.
The head of Red Crescent said that it was "impossible" for anyone to have survived the crash.
All direct flights between Ukraine and Iran were suspended, while a number of airlines announced that they were avoiding both Iranian and Iraqi airspace.
Air France and KLM said they would use alternative routes while Lufthansa said it was also cancelling its daily flight to Tehran. Qantas, Air India, Japan Airlines, Singapore Airlines and Malaysia Airlines were among other airlines taking action.
Who was on board?
Among the victims were 82 Iranians, 63 Canadians, 11 Ukrainians including all nine crew, 10 Swedes, four Afghans, three Britons and three Germans, Ukrainian Foreign Minister Vadym Prystaiko said. Fifteen of the dead were children.
Ukrainian officials said that 169 people had bought tickets for the flight but two had not boarded the plane.
The German government later said "we currently have no knowledge that German citizens are among the victims of the plane crash in Iran".
It was unclear why such a large number of Canadians were on board, however the airline provides relatively inexpensive flights via Kyiv to Toronto. Some of the Canadians were thought to be students returning at the end of the school holidays.
Prime Minister Justin Trudeau said he was "shocked and saddened" by the news of the crash, offering "deepest condolences to those who have lost family, friends, and loved ones in this tragedy".
Heartbreaking news from Tehran this morning: 176 people, including 63 Canadians, have lost their lives in a tragic plane crash. My thoughts are with all those who are mourning the loss of family members, friends, and loved ones. https://t.co/rSTTbGj0oG— Justin Trudeau (@JustinTrudeau) January 8, 2020
Sweden's foreign ministry said its embassy in Tehran was seeking further information about the crash.
Our deepest condolences, which I have also conveyed personally to Ukraine’s Foreign Minister @VPrystaiko, on the tragic plane accident in Teheran. Our thoughts are with the victims and their loved ones.— Ann Linde (@AnnLinde) January 8, 2020
Iran's head of emergency operations said 147 of the victims were Iranian. That would suggest that 65 of the foreign nationals had dual nationalities. The Ukrainian airline gave a helpline number for further information about passengers: +38-044-581-50-19.
Ukraine's President Volodymyr Zelensky said he was cutting short a trip to Oman and flying back to Kyiv.
"My sincere condolences to the relatives and friends of all passengers and crew," he said in a statement.
The airline named the nine crew killed in the crash, including three pilots and six flight attendants.
Among those who died was Valeriia Ovcharuk, a 28-year-old who posted pictures on social media of several of the destinations she had visited with the airline, from Lithuania to Bangkok and Beijing.
There was reportedly no sign of any problems with the plane before take-off and the airline's president said it had an "excellent, reliable crew". UIA has never had a fatal crash before.
Mr Zelensky said Ukraine's entire civil aviation fleet would be checked for airworthiness and criminal proceedings would be opened into the disaster.
This type of Boeing is one of the most widely-used commercial airliners in the world. Aviation safety analyst Todd Curtis told the BBC: "The airplane was heavily fragmented which means either there was an intense impact on the ground or something happened in the sky."
There are several thousand Boeing 737-800s in operation around the world which have completed tens of millions of flights. They have been involved in 10 incidents, including this crash, where at least one passenger was killed, Mr Curtis said.
The plane's engines are made by French-US firm CFM, owned by General Electric and France's Safran. Mr Curtis said Iranian, Ukrainian, US and French authorities would all be involved in the investigation, but it was unclear how they would work together. Iran is currently under US sanctions and there are severe tensions between the two countries.
Boeing expressed its condolences to everyone involved.
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. pic.twitter.com/TsTlyY34Vd— The Boeing Company (@Boeing) January 8, 2020
Todd Curtis said there nothing at this stage pointed to any particular cause. "One cannot discount the possibility that something outside the aircraft, a mid-air collision or some other issue, may have been involved."
What went wrong?
Flight data from the Ukrainian Airlines Boeing 737-800 is openly available online. It shows that the plane climbed normally after taking-off from Tehran.
It reached nearly 8,000ft (2,400m) before the aircraft's data suddenly disappears.
This is unusual and would suggest some type of catastrophic incident on board the plane. We have no evidence at this stage to tell us what caused the incident.
According to a former air crash investigator, any suggestion of engine failure feels premature. This possibility can't be ruled out at this early stage but an airliner such as the Boeing 737-800 is designed to keep flying if there is an engine failure.
Plus, if there was a failure then we would normally expect the flight data to show the plane's climb becoming less steep.