Authorities in Ukraine are not ruling out that one of the country's planes, which crashed early on Wednesday in Iran killing all 176 passengers and crew, was brought down by a missile or an attack amid a sharp escalation of tension between Tehran and Washington.

The Ukrainian International airliner bound for the Ukrainian capital, Kyiv, plunged from the sky minutes after takeoff from the Imam Khomeini International Airport in Tehran.

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The Boeing 737-800 was last seen on radar at 2,400 metres, according to the FlightRadar 24 monitoring website, hours after Tehran launched more than a dozen ballistic missiles at US facilities in neighbouring Iraq.

"We must investigate all possible causes," Ukraine's President Volodymyr Zelenskyy wrote on Facebook, shortly after Iranian officials said the crash was caused by a fire that struck one of the plane's engines, causing the pilots to lose control.

Zelenskyy has ordered Ukrainian prosecutors to open an investigation into the crash.

Shortly before Zelenskyy's announcement, Ukraine's embassy in Iran removed a statement that ruled out the attacks from its website, Ukrainian media reported.

Relatives, colleagues, and friends pay their respects to the UIA crew who died when their plane crashed shortly after take off from Tehran on its way to Kyiv [Sergey Dolzhenko/EPA]

The Ukrainian UNIAN news agency quoted the Jordanian Al Hadath news outlet as claiming that the plane had been shot down by Iranian air force by mistake.

"Until the official investigation is over, naming any version would be manipulation," Prime Minister Oleksiy Honcharuk told a news conference in response to a question about the possibility of a missile or attack on the plane.

Canada offers expertise

He said a group of experts were heading to Iran to investigate the crash, collect victims' remains and return them to Ukraine.

"We're getting the group ready for departure," he said.

According to international agreements, Ukraine can participate in the investigation as the state of registry and operator.

Most of the passengers on board the plane were from Iran, but there were also 63 Canadian nationals, and 138 of those on board had onward connections to Canada. The Tehran-Toronto route via Kyiv route was popular with Canadians of Iranian descent

The country's Prime Minister Justin Trudeau said Canada, which broke off diplomatic relations with Iran in 2012, should play a significant role in any investigation into the crash.

"Canada is one of a handful of countries with a high degree of expertise when it comes to these sorts of accidents and therefore we have much to contribute," Trudeau said.

"I am confident that in our engagement both through our allies and directly, we are going to make sure that we are a substantive contributor to this investigation," he told a news conference in Ottawa, adding that the foreign minister of Canada would talk to his Iranian counterpart to underline the need for a "thorough investigation." It was "dangerous to speculate" on the possible causes so early in the investigation, he added.

Iranian officials said the flight data and cockpit voice recorders, known as "black boxes," which will be crucial to establishing the cause of the crash, had been found.

“The rumours about the plane are completely false and no military or political expert has confirmed it,” General Abolfazl Shekarchi, spokesman for the Iranian armed forces, was quoted by the semi-official Fars news agency as saying. He said the speculation was "psychological warfare" by the government's opponents.

Under international rules, responsibility for investigating the crash lies with Iran. Ali Abedzadeh, head of the Iran Civil Aviation Organization, earlier said that Tehran would not hand over to the US or Boeing the recovered black boxes.

Ukraine International airline said the three-year-old plane underwent a maintenance check on Monday and no technical problems has been identified.

"It was one of our best airplanes with a great, reliable crew," the company's CEO Yevhen Dykhne told a news conference in Kyiv through tears.

Challenging airport conditions

His colleague said that flight conditions at Tehran's airport were complicated.

"Knowing that the Tehran airport is not an easy one, our flight training for all crews on [Boeing] 737s were held exclusively at the Tehran airport," Ihor Sosnovsky, the airline's vice president, told reporters.

Considering that the plane was 2,400 metres above ground before it disappeared off radar, crew error "was unlikely", Dykhne said.

One of the engines from the crashed plane lies among the wreckage outside Tehran. [Abedin Taherkenareh/EPA]

He refused to comment on whether the plane was shot down by a missile.

In Washington, a Democrat who attended a classified briefing from Trump administration officials on Capitol Hill - including Defense Secretary Mark Esper, Secretary of State Mike Pompeo and CIA Director Gina Haspel - said the briefers had no intelligence indicating the plane was shot down. The lawmaker spoke on condition of anonymity.

Five security sources - three Americans, one European and one Canadian - who asked not to be named, told Reuters the initial assessment of Western intelligence agencies was that the plane had suffered a technical malfunction and had not been brought down by a missile. There was evidence one of the jet's engines had overheated, the Canadian source said

US aircraft surveillance firm Aireon said it would be sharing position data from the jet with appropriate authorities. The company's satellite-based global tracking system provides more detailed information than is available on commercial websites like FlightRadar24

Chief pilot Volodymir Haponenko had flown more than 10,000 hours during hundreds of flights, UIA said.

"Aviation was his life, he was a great pilot," his widow Yekaterina Haponenko, a mother of two, said in televised remarks.

Eleven Ukrainian nationals, including nine crew, were among the victims, Ukraine's Foreign Minister Vadym Prystaiko said.

As well as the 82 Iranians and 63 Canadians, there were 10 passengers from Sweden, four from Afghanistan, three Germans and three Britons.

Officials in Tehran said 147 of the victims were Iranian, which suggests many had dual citizenship.

Boeing's 737-800 planes will face renewed international scrutiny after the crash.

In December, Boeing's CEO Dennis Muilenburg was removed after one of the company's most disastrous years. Two 737 Max planes crashed in 2018, and all Max planes worldwide have been grounded.

Mansur Mirovalev contributed to this report from Kyiv