Ukraine's top security official has said investigators believe the cockpit of a downed Ukraine International Airlines Boeing 737-800 airliner was hit by an Iranian missile from below, killing the pilots instantly.

Key points: Ukraine claims it had evidence of Iran's fault for the crash before its admission

Ukraine claims it had evidence of Iran's fault for the crash before its admission The Ukrainian Government will compensate families whose relatives died

The Ukrainian Government will compensate families whose relatives died Iranians are protesting over the Government's initial denial of fault

Ukraine's National Security and Defence Secretary Oleksiy Danilov has spoken about images and video of the wreckage taken by Ukrainian officials in Iran investigating the crash, which killed all 176 people onboard.

"It hit the cockpit from underneath. We think this is proof," Mr Danilov told the BBC, referring to a photo of the downed cockpit taken by Ukrainian investigators.

"It explains why we didn't hear anything from the pilots. They died immediately after the first hit."

Images released by the Ukrainian Presidential Press Office show the wreckage of the downed Boeing 737-800. ( Ukrainian Presidential Press Office )

Ukrainian officials said they did not make evidence public earlier due to concerns about being deported from Iran. ( Ukrainian Presidential Press Office )

Another photo presented by Mr Danilov appeared to show remnants of the missile that allegedly downed the airliner.

"We think this is part of the missile," Mr Danilov said, adding that the evidence had been collected before Iran admitted fault for shooting down the passenger plane.

"As soon as the Iranians gave us access to these items, our specialists kept us updated every hour as to what was happening. And very quickly as we gathered information, we understood what had happened," he said.

A still shows what Ukrainian investigators believe is part of the missile that downed flight PS752. ( Supplied: BBC )

Iran initially attributed the crash to mechanical faults and a pilot losing control, but there was early speculation from some experts that the plane could have been shot down.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 19 seconds 19 s Footage appears to show a mid-air explosion close to the flight's last transponder position.

Days later, the United States, Canada and Australia said they had intelligence indicating that was the case.

"We couldn't make [the evidence] public right away. We still needed to be able to work there," Mr Danilov said.

"Iran is a very difficult country as you know. And we were worried that they could send our specialists back."

Mr Danilov added: "I think Iran understood that it had no choice [but to admit fault for the crash].

"Even if they had tried to create obstacles, we already had enough to show the international community what really happened here," he said.

Decision to keep airspace open criticised

Ukraine International Airlines vice-president Ihor Sosnovskiy has called Iran's decision to keep civilian airspace open amid hostilities with the US "absolutely irresponsible".

The crash came just hours after Iran launched a ballistic missile attack on Iraqi bases housing US soldiers.

"When you act in war, then you act however you wish. But there must be protection around ordinary people," he told a media conference.

"If they are shooting somewhere from somewhere, they are obliged to close the airport."

Ukraine's President Volodymyr Zelenskiy has said the country expects a full investigation, a full admission of guilt and compensation.

"Iran has pleaded guilty to crashing the Ukrainian plane. But we insist on a full admission of guilt," Mr Zelenskiy said.

"We expect from Iran assurances of their readiness for a full and open investigation, bringing those responsible to justice, the return of the bodies of the dead, the payment of compensation, official apologies through diplomatic channels."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 27 seconds 27 s Debris litters the crash site

Nevertheless, Ukraine, Canada, Germany and the UK have called Iran's admission of fault an important first step.

"I spoke with President [Hassan] Rouhani of Iran. I told him that Iran's admission that its own armed forces unintentionally shot down flight 752 is an important step towards providing answers for families, but I noted that many more steps must be taken," Canadian Prime Minister Justin Trudeau told a media conference in Ottawa.

"What Iran has admitted to is very serious. Shooting down a civilian aircraft is horrific. Iran must take full responsibility."

British Prime Minister Boris Johnson said Iran's admission was "an important first step" and that it was "vital that all leaders now pursue a diplomatic way forward" to avoid conflict.

Iran repeatedly denied Western accusations that it was responsible for the crash. ( AP: Ebrahim Noroozi )

Protests against authorities have spread across Iran including in the capital Tehran, Shiraz, Esfahan, Hamedan and Orumiyeh, reportedly calling for supreme leader Ayatollah Khamenei to resign.

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US President Donald Trump, who has said he does not seek "regime change" in Iran, took to Twitter to express his support for the demonstrators, writing: "We are following your protests closely, and are inspired by your courage."

"The Government of Iran must allow human rights groups to monitor and report facts from the ground on the ongoing protests by the Iranian people. There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching," Trump wrote.

Mr Zelenskiy has said his Government will provide more than $8,000 in financial compensation to the families of its citizens who perished in the crash, and will also push Iran to provide separate compensation.

Mr Zelenskiy said he had also spoken on the phone with Mr Rouhani, who promised to prosecute those responsible.

"We will return all those dead to their families," Mr Zelenskiy said.

"All the culprits will be punished."

ABC/wires