Dubai: Iran said on Saturday its military had mistakenly shot down a Ukrainian plane killing all 176 aboard, saying air defences were fired in error while on high alert in the tense aftermath of Iranian missile strikes on US targets in Iraq.

Iran had previously vigorously denied bringing the plane down. Supreme Leader Ali Khamenei, who until Saturday kept silent about the crash, said information should be made public.

Wednesday's crash heightened international pressure on Iran after months of friction with the United States and tit-for-tat attacks. A US drone strike had killed a top Iranian military commander in Iraq on January 3, prompting Tehran to fire at US targets on Wednesday.

Canada, which had 57 citizens on board, and the United States had both said they believed an Iranian missile brought down the aircraft although they said it was probably an accident.

Canada's foreign minister had told Iran "the world is watching."

"The Islamic Republic of Iran deeply regrets this disastrous mistake," Iranian President Hassan Rouhani wrote on Twitter, promising that those behind the incident would be prosecuted. "My thoughts and prayers go to all the mourning families."

Experts said mounting international scrutiny would have made it all but impossible to hide signs of a missile strike in an investigation and Iran may have felt a policy U-turn was better than battling rising criticism abroad and growing grief and anger at home.

Many of the victims were Iranians with dual nationality.

In Twitter messages, angry Iranians asked why the plane was allowed to take off with tensions so high after the Iranian military action. The plane came down just hours after Iran launched rockets to attack US troops in Iraqi bases, at a time when Tehran was on high alert for possible reprisals.

Responding to Iran's announcement, Ukrainian President Volodymyr Zelenskiy said he wanted an official apology and full cooperation, demanding those responsible be held to account.

Iran's Foreign Minister Mohammad Javad Zarif wrote on Twitter that "human error at time of crisis caused by U.S. adventurism led to disaster," citing an initial armed forces investigation into the crash of the Boeing 737-800.

Tell-Tale Signs

A military statement said the plane flew close to a sensitive military site of the elite Revolutionary Guards. At the time, planes had been spotted on radar near strategic sites leading to "further alertness" in air defences.

Ukrainian Foreign Minister Vadym Prystaiko had said on Friday that the plane was following a normal flight corridor.

Experts said a probe would almost certainly have revealed tell-tale signs on the fuselage remains of a missile strike.

"There's nothing you can do to cover it up or hide it," said Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University and former U.S. National Transportation Safety Board investigator. "Evidence is evidence."

Former U.S. Federal Aviation Administration accident investigator, Mike Daniel, told Reuters: "When the facts and evidence started coming out, I think it was inevitable that the government of Iran (would) accept culpability."

The White House did not immediately respond to a request for comment.

Mobile phone footage posted and circulated by ordinary Iranians on Twitter after the crash had indicated that it came down in flames and exploded as it struck the ground.

Iran had said on Thursday it would download the information from voice and flight data recorders, known as black boxes, to determine what had happened, although it had said the process could take one to two months.

Tehran had said it could ask Russia, Canada, France or Ukraine for help in the investigation.

Public Grief

In its initial denials this week, Iran had described accusations that a missile was to blame as "psychological warfare".

Grief-stricken Iranians complained on social media that Iran's authorities had spent more time fending off criticism than sympathising with victims' families.

"Why were any civilian airlines flying out of Tehran airport in those conditions?" Twitter user Shiva Balaghi wrote.

Former vice president Shaheendokht Molaverdi said on Twitter: "We hope one official will resign in order to calm down angry people who think that they have been humiliated."

A US official had said data showed the plane airborne for two minutes after departing Tehran when heat signatures of two surface-to-air missiles were detected. There was an explosion in the vicinity and heat data showed the plane on fire as it fell. U.S. military satellites detect infrared emissions from heat.

US President Donald Trump had said "somebody could have made a mistake."

The disaster had echoes of an incident in 1988, when the U.S. warship USS Vincennes shot down an Iranian airliner, killing 290 people. Washington said it was a tragic accident. Tehran said it was intentional.