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By Aresu Eqbali in Tehran, Rory Jones in Dubai and Georgi Kantchev in Moscow 

The Ukraine International Airlines flight that crashed after takeoff from Tehran was on fire before it hit the ground, Iran's aviation regulator said, as investigators remained open to the possibility that the jet was hit by a projectile or an explosion.

The Boeing Co. 737-800 single-aisle jet ascended to 8,000 feet before disappearing from radar and crashing, killing all 176 people on board, Ali Abedzadeh, head of Iran's Civil Aviation Organization said, according to the country's state news agency.

The plane was aflame as its hit the ground and exploded, Mr. Abedzadeh said, citing witnesses on the ground and a flight crew on another aircraft at a higher altitude.

The cause of the fire is unclear but investigators are considering a number of possible scenarios, including a strike by an antiaircraft missile, a collision with a drone, and an explosion of an engine or inside the aircraft, said Oleksiy Danilov, the head of Ukraine's National Security and Defense Council, in a Facebook post on Thursday.

He didn't mention any evidence pointing to such an attack.

A Ukrainian security official cautioned that these are preliminary versions investigators are considering and no conclusions have been reached. "Too many things are unclear at the moment," the official said.

Ukraine International Airlines didn't immediately respond to a request for comment.

A spokesman for Iran's aviation regulator couldn't immediately be reached for comment on Mr. Danilov's statement.

Earlier, Mohammad Eslami, Iran's minister of roads and urban development, told news agency ISNA, "Rumors about the incident being a terrorist attack, explosion or shooting at the plane...are not true."

No radio messages were received from the pilot about unusual conditions, Mr. Abedzadeh added in an initial report, according to IRNA, the state news agency.

The investigation is expected to be one of the most politically thorny probes ever. Tensions are high between the U.S. and Iran over the killing of a top Iranian general. Boeing is already dealing with the fallout from the grounding of its 737 MAX fleet after two fatal crashes involving that jet.

Boeing said it is in contact with Ukraine International Airlines and is ready to assist in any way. The U.S. State Department said it is prepared to offer Ukraine all possible assistance and called for cooperation in the investigation.

A team of 45 Ukrainian experts and officials arrived in Tehran in the early hours of Thursday and will be involved in decoding the plane's black boxes and identifying and repatriating bodies. The team has already collected DNA samples from relatives of the victims in Ukraine.

"The priority for Ukraine is to establish the causes of the crash," Ukrainian President Volodymyr Zelensky said in a video posted on his official Facebook page Thursday. "To do this, a detailed and independent investigation will be carried out in accordance with international law."

An unverified video circulating on social media appeared to show a plane on fire before diving to the ground and crashing in an explosion that lighted up the night. In the video, a person can be heard saying, "The plane is on fire...God help! Let's call firefighters."

The person in the video gives his location as Ferdosiye, northwest of Imam Khomeini International Airport, from where the Ukrainian airliner departed.

The crash came hours after Iran launched missile attacks on U.S. troops at two Iraqi bases in retaliation for the killing of Iranian Maj. Gen. Qassem Soleimani. The timing of those attacks had raised questions about whether the Ukrainian airliner was downed by a projectile.

In response to the Iranian strikes in Iraq, and ahead of the crash, the Federal Aviation Administration said it would prohibit U.S. carriers from traveling in airspace over Iran, Iraq and the Persian Gulf. U.S. carriers don't fly to the Persian Gulf region, but some flights skirt Iraq and Iran on the way to Asia.

Turkish Airlines and Qatar Airways both departed Imam Khomeini International Airport after the Iranian strikes and before the Ukraine International Airlines flight that crashed, according to FlightRadar24, a flight tracking site. Since the strikes on the military bases in Iraq, carriers including Air France, KLM and Qantas have said they would avoid Iranian and Iraqi airspace.

Ukraine International Airlines said Wednesday that it hadn't received any security alerts before the plane took off. It has since ceased flights to Iran indefinitely.

Mr. Zelensky said other international planes were landing and taking off at the Tehran airport in the morning of the crash and only the U.S. had banned flights. He recommended that Ukrainian citizens avoid the region and declared Thursday a day of national mourning.

Mr. Zelensky said that he would speak by telephone with Iranian President Hassan Rouhani to discuss the investigation and urged the international community to join the probe.

It isn't clear how much access U.S. investigators will get to the crash site. Hassan Rezaifar, director general of the ministry of road and urban development for the bureau of accidents investigation, said Thursday that the probe would be conducted to international standards set by the Montreal-based International Civil Aviation Organization.

That would indicate U.S. companies and entities could be involved. But Mr. Abedzadeh, the head of Iran's aviation regulator, also told the semiofficial Mehr News Agency on Wednesday that his organization wouldn't provide Boeing or the U.S. access to the black box.

Based on the ICAO convention, the home country of the airline and the makers of the plane and its major systems are entitled to appoint a representative to participate in crash probes.

CFM International, a joint venture between General Electric Co. and France's Safran SA that makes the engine for the 737-800, could also seek access to the site. CRM said Wednesday it is saddened by the incident and "any speculation regarding the cause is premature."

Write to Rory Jones at rory.jones@wsj.com and Georgi Kantchev at georgi.kantchev@wsj.com

(END) Dow Jones Newswires

January 09, 2020 05:27 ET (10:27 GMT)

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