An Iranian commander says repeated requests to close the country's airspace were denied before a civilian passenger plane was "unintentionally" shot down outside Tehran.

Key points: Iranian authorities have said those responsible for the crash will be held accountable

Iranian authorities have said those responsible for the crash will be held accountable The Boeing 737 went down during take-off from Tehran just hours after Iran attacked US forces

The Boeing 737 went down during take-off from Tehran just hours after Iran attacked US forces Officials say 82 Iranians, at least 57 Canadians and 11 Ukrainians died in the crash

After initially denying responsibility, Iran has admitted to downing a Ukraine International Airlines Boeing 737-800 bound for Kiev amid US hostilities in the region on Wednesday, killing all 176 people on board.

General Amir Ali Hajizadeh, commander of Iran's Islamic Revolutionary Guard Corps airspace unit, said in a press conference his unit accepted "full responsibility" for the incident.

"We had requested several times that the country's airspace become clear of all flights," he said.

"Requests were made but due to some considerations it was not done and at the same time with the flights, the war situation continued to exist."

In an address broadcast by state TV, he said that when he learned about the downing of the plane: "I wished I was dead."

All 176 passengers on board the Ukrainian plane died in the incident. ( AP: Ebrahim Noroozi )

"We are sorry about this incident and we consider ourselves mournful just like families of these martyrs and sympathise with them," he said.

"It is as such that we have lost our own loved ones and we are regretful about it."

He said the airline's pilot and crew had done nothing wrong, and instead an Iranian officer made the "bad decision" to open fire on the plane after mistaking it for a cruise missile.

But General Hajizadeh blamed the US for the tragedy.

"After all, this is the price of mischiefs, turbulences and actions of America in the region," he said.

"That night … the probability of fighter jets and cruise missiles entering the country was very high and [we] had prepared ourselves for an all-out conflict."

Iran's Supreme Leader has offered condolences and called for an investigation into the incident.

Iran's semi-official Fars news agency reported that Ayatollah Ali Khamenei — who until Saturday kept silent about the crash — had ordered top security officials to review the crash and announce the results.

"If some individuals, in any position, were aware of the issue but made statements contradicting the reality or hid the truth for any reason, they should be named and tried," said Fars, which is close to the Guard.

Others speculated that the security forces may have concealed information from civilian authorities.

"Concealing the truth from the administration is dreadful," Mohammad Fazeli, a sociology professor in Tehran, wrote on social media.

"If it had not been concealed, the head of civil aviation and the government spokesmen would not have persistently denied it."

The plane crashed on shortly after take-off from Tehran's main airport. ( AP: Mohammad Nasiri )

'Iran must take full responsibility'

Even as top Iranian officials and the military issued apologies, protests against authorities spread across Iran including in the capital Tehran, Shiraz, Esfahan, Hamedan and Orumiyeh.

Britain late on Saturday accused Iran of violating international law after its ambassador was arrested and briefly detained during the protests in Tehran.

Iranian media said ambassador Rob Macaire was arrested outside a university and detained for more than an hour before being released.

UK Foreign Secretary Dominic Raab said "the arrest of our ambassador in Tehran without grounds or explanation is a flagrant violation of international law".

Foreign governments have condemned Iran's actions in shooting down the plane, with Ukraine demanding compensation.

Canada, Ukraine and Britain, however, called Tehran's admission 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.

"Canada will not rest until we get the accountability, justice, and closure that the families deserve."

Mr Trudeau said Mr Rouhani committed to collaborating with Canadian investigators, working to de-escalate tensions in the region and continuing a dialogue.

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.

"Our profound regrets, apologies and condolences to our people, to the families of all victims, and to other affected nations."

President Rouhani tweeted the country "deeply regrets this disastrous mistake," promising that those behind the incident would be prosecuted.

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.

'Human error' to blame

Earlier on Saturday (AEDT), Iranian Foreign Minister Javad Zarif tweeted that "human error" was to blame for the incident, after initially denying Iran brought down the aircraft in the tense aftermath of missile strikes on US targets in Iraq.

A statement read on state television said the airliner had flown close to a sensitive military site and parties responsible for shooting down the jet would be held accountable.

The statement said the plane was mistaken for a "hostile target" after it turned toward a "sensitive military centre" of the Revolutionary Guard.

The military was at its "highest level of readiness" it said, amid the heightened tensions with the US.

"In such a condition, because of human error and in a unintentional way, the flight was hit," the statement said.

It apologised for the disaster and said it would upgrade its systems to prevent such "mistakes" in the future.

The jetliner went down on the outskirts of Tehran during take-off, just hours after Iran launched a barrage of missiles at US forces.

Memorials for those killed in the plane crash have been held across Canada. ( AP: Rob Gurdebeke/The Canadian Press )

Iran initially denied any wrongdoing but the US and Canada, citing intelligence, said they believed Iran shot down the aircraft.

The plane, en route to the Ukrainian capital of Kiev, was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 57 Canadians and 11 Ukrainians, according to officials.

The Canadian Government earlier lowered the nation's death toll from 63.

"This is the right step for the Iranian Government to admit responsibility, and it gives people a step toward closure with this admission," said Payman Parseyan, a prominent Iranian-Canadian in western Canada who lost a number of friends in the crash.

"I think the investigation would have disclosed it whether they admitted it or not. This will give them an opportunity to save face."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Canadian Prime Minister Justin Trudeau said he wanted the crash "thoroughly investigated".

Australian Foreign Minister Marise Payne offered her condolences to the victims and their families and said Iran's "acknowledgement and apology to all those affected was welcome".

Moving forward, she said it would be important that "a full and transparent investigation is undertaken, involving all nations who lost citizens in the tragedy".

"It is essential that all questions about how this tragedy happened are answered fully and that confidence is rebuilt in the safety of the international civilian air traffic network," she added.

Television reports on Friday indicated that debris had been cleared from the crash site, leaving the area to scavengers to pick over.

If the wreckage was indeed moved, some clues might have been lost unless the Iranians took careful steps to preserve evidence.

"Normally you would very carefully to map out a debris field," said Steve Wallace, former head of the accident-investigations office of the Federal Aviation Administration.

"If a missile struck the airplane, you would expect to find some pieces of it and residue of the explosive."

AP/Reuters