The crew of a Ukrainian jetliner that crashed in Iran, killing 176 people, never made a radio call for help and were trying to turn back for the airport when their burning plane went down, an initial Iranian report said on Thursday.

The Iranian report suggests a sudden emergency struck the Boeing 737 operated by Ukrainian International Airlines in the early hours of Wednesday morning, when it crashed, just minutes after taking off from Imam Khomeini International Airport in Tehran. It was the first fatal crash of the country's largest carrier, Ukraine International Airlines.

Investigators from Iran's Civil Aviation Organisation offered no immediate explanation for the disaster, however.

Ukraine, meanwhile, said it considered a missile strike or terrorism as possible theories for the crash, despite Iran's denials.

Iranian officials initially blamed a technical malfunction for the crash, something initially backed by Ukrainian officials before they said they wouldn't speculate amid an ongoing investigation.

The crash came just a few hours after Iran launched a ballistic missile attack against Iraqi military bases housing US troops amid a confrontation with Washington over it killing an Iranian Revolutionary Guard general in a drone strike last week.

The Ukrainian International Airlines took off at 6:12am on Wednesday, after nearly an hour's delay at Tehran's Imam Khomeini Airport, the main airport for travellers in Iran. It gained altitude heading west, reaching nearly 8,000 feet, according to both the report and flight-tracking data.

Then something went wrong, though no radio messages were received from the pilot regarding unusual situations, the report said. In emergencies, pilots typically immediately contact air-traffic controllers.

Eyewitnesses, including the crew of another flight passing above it, described seeing the plane engulfed in flames before crashing at 6:18am, the report said.

The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the flight to Kyiv, Ukraine.

The report also confirmed that both of the so-called black boxes that contain data and cockpit communications from the plane had been recovered, though they sustained damage and some parts of their memory was lost.

It also said that investigators have initially ruled out laser or electromagnetic interference as causing the crash.

Oleksiy Danilov, secretary of Ukraine's Security Council, told Ukrainian media that officials had several working theories regarding the crash, including a missile strike.

“A strike by a missile, possibly a Tor missile system, is among the main [theories], as information has surfaced on the internet about elements of a missile being found near the site of the crash,” Danilov said.

He did not elaborate on where he saw the information on the internet.

Some 45 Ukrainian investigators, who arrived in Iran earlier on Thursday, currently await permission from Iranian authorities to examine the crash site and look for missile fragments, Danilov said. He added that other possible causes under consideration included a drone or another flying object crashing into the plane, a terrorist attack or an engine malfunction causing an explosion.

Iran did not immediately respond to the Ukrainian comments.

However, Gen Abolfazl Shekarchi, the spokesman of the Iranian armed forces, denied a missile hit the airplane in comments reported on Wednesday by the semiofficial Fars news agency.

He dismissed the allegation as "psychological warfare" by foreign-based Iranian opposition groups.

Ukrainian officials, for their part, initially agreed with Iranian suspicions that the three-year-old plane was brought down by mechanical trouble but later backed away from that and declined to offer a cause while the investigation is going on.

Ukrainian President Volodymyr Zelenskiy said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.

While the cause of the tragedy remained unknown, the disaster could further damage Boeings reputation, which has been battered by the furore over two deadly crashes involving a different model of the Boeing jet, the much-newer 737 Max, which has been grounded for nearly 10 months.

The uproar had led to the firing of the company's CEO last month.

Boeing extended condolences to the victims families and said it stands ready to assist.

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians and 11 Ukrainians, according to officials.

Many of the passengers were believed to be international students attending universities in Canada; they were making their way back to Toronto by way of Kyiv after visiting with family during the winter break.

Canadian Prime Minister Justin Trudeau said 138 of the passengers were bound for Canada.

The flight also included a family of four and newlyweds, too. The manifest listed several teenagers and children, some as young as 1 or 2.

The crash ranked among the worst losses of life for Canadians in an aviation disaster.

The flag over Parliament in Ottawa was lowered to half-staff, and Trudeau vowed to get to the bottom of the disaster.

"Know that all Canadians are grieving with you," he said, addressing the victims families.

Ukraine observes day of mourning

Following the devastating crash, Ukraine's President Zelensky announced a day of national mourning on Thursday.

"January 9 has been decreed a day of national mourning," the Ukrainian leader said in a video address posted on his official Facebook account.

Zelensky had on Wednesday cut short a trip to Oman and flew back to Ukraine after the plane crash.

"Ukraine's priority is to establish the causes of the catastrophe. We are undoubtedly going to get to the truth," Zelensky said.

The Ukranian security officials, who are visiting Iran to participate in the investigation, will also seek to identify the Ukrainian victims with a view to repatriating their remains.

Those who died on Flight PS752 were mostly Iranian and Canadian nationals, but they also included 11 Ukrainians.

These included nine crew members, whose photos were displayed at the departure hall in the Borispyl airport in a tribute.

The president's office issued a photo of Zelensky laying a bouquet of roses at the impromptu memorial.

He called on the public to "refrain from all manipulation, speculation and conspiracy theories" about the crash, which occurred just minutes after takeoff on the same morning as Iran's missile strike against US forces in Iraq.