Iran said on Saturday its military had mistakenly shot down a Ukrainian plane killing all 176 aboard, saying air defenses were fired in error while on alert in the tense aftermath of Iranian missile strikes on US targets in Iraq. Iran had denied for days after Wednesdays crash that it brought down the plane, although a top Revolutionary Guards commander said on Saturday that he had informed the authorities about the unintentional missile strike the same day it happened. Supreme Leader Ali Khamenei, who until Saturday had kept silent about the crash, said information should be made public, while top officials and the military issued apologies. But state television suggested revealing the truth about what happened might be used by "enemies of Iran", usually a reference to the United States and Israel. The 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 Jan. 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 error. "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 any investigation and Iran may have felt a U-turn was better than battling rising criticism abroad and growing grief and anger at home, as many victims were Iranians with dual nationality. In Twitter messages, angry Iranians asked why the plane was allowed to take off with tensions in Iran so high. The plane came down when Tehran was alert for US reprisals in the hours after Iran launched rockets at US troops in Iraqi bases. THOROUGH INVESTIGATION Irans Revolutionary Guards, in a rare admission of error, apologized to the nation and accepted full responsibility. Senior Guards commander Amirali Hajizadeh said he had informed Irans authorities on Wednesday about the unintentional strike, a comment that raised questions about why officials had publicly denied it for so long. Speaking on state television, he said he wished "I could die" when he heard the news about the incident. Responding to Irans announcement on Saturday, Ukraine demanded an official apology and compensation. Canadian Prime Minister Justin Trudeau called for "a complete and thorough investigation" with Irans full cooperation. Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter that "human error at time of crisis caused by US adventurism led to disaster", citing an initial armed forces investigation into the crash of the Boeing 737-800. A military statement said the plane flew close to a sensitive Revolutionary Guards site a time of high alert, although Ukraine said the plane was in a normal flight corridor. Ukraine International Airlines said Iran should have closed the airport. Its vice president said the carrier received no indication it faced a threat and was cleared for take-off. Aviation experts said it is up to a country to close its airspace when there is a risk. Kiev and Paris agreed on Saturday that French specialists would help decode the black boxes, the office of Ukrainian President Volodymyr Zelenskiy said. The French air accident agency analyzed data from a Boeing crash in Ethiopia last year. Iran has said it would download information from the voice and flight data recorders itself but would seek help if needed. Analysts said a probe would almost certainly have revealed signs on the smashed fuselage of a missile strike. "Theres 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 US National Transportation Safety Board investigator. "Evidence is evidence." IRANIAN COMPLAINTS Mobile phone footage posted and circulated by ordinary Iranians on Twitter after the crash indicated the plane came down in flames and exploded as it struck the ground. Grief-stricken Iranians complained on social media that Irans authorities had spent more time fending off criticism than sympathizing with victims families. "Why were any civilian airlines flying out of Tehran airport in those conditions?" Twitter user Shiva Balaghi wrote. A US official said data showed the plane was 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. US military satellites detect infrared emissions from heat. The disaster echoed of a 1988 incident, when a US warship shot down an Iranian airliner, killing 290 people. Washington said it was an accident. Tehran said it was intentional. INTERNATIONAL REACTIONS President Volodymyr Zelensky demanded Saturday that Iran punish those responsible for the downing of a Ukrainian airliner, pay compensation and apologize. "We expect Iran... to bring the guilty to the courts," the Ukrainian leader wrote on Facebook, calling also for the "payment of compensation" and the return of remains. "We hope the inquiry will be pursued without deliberate delay and without obstruction," Zelensky added He urged "total access" to the full inquiry for 45 Ukrainian experts, and in a tweet also sought an "official apology". UIA chief Yevhenii Dykhne also posted on Facebook, saying: "We never had any doubt that our crew and our plane could be the cause of this terrible catastrophe. They were the best." Ukraine said Friday its experts dispatched to Iran had been granted access to the flights black boxes, debris from the plane, the crash site, and recordings of conversations between the pilot and the airport control tower. The majority of passengers on UIA Flight PS752 from Tehran to Kiev were Iranian-Canadian dual nationals but also included Ukrainians, Afghans, Britons, and Swedes Canadian Prime Minister Justin Trudeau says his focus will be on justice for the families of those who died when Iran shot down a Ukrainian jetliner earlier this week. Trudeau issued a statement Saturday saying Canada is concentrating on "closure, accountability, transparency, and justice for the families and loved ones of the victims." Trudeau called the shootdown a national tragedy and says "all Canadians are mourning together." He says they will continue to work with partners around the world to ensure a complete and thorough investigation. He says the Canadian government expects full cooperation from Iranian authorities. For his part, Swedens prime minister says that it is demanding Iran cooperate without any restrictions in investigating the downing of the jetliner. Stefan Lofven said Saturday: "The fact that a plane was shot down is terrible and horrifying. To have a civil aircraft shot down - whether accidentally or not - is an act that must be condemned and Iran must take full responsibility also in relation to those affected." A total of 10 people with Swedish citizenship and another seven residing in Scandinavian country are believed to have been aboard the ill-fated Ukrainian International Airlines flight 752 flying out of the Iranian capital. Sweden said it would remain in close contact with the other countries which lost their citizens in the crash. Lofven said a foreign minister-level group was formed Friday to follow up with the planes downing and its investigation. Also, Germanys foreign minister on Saturday called on Iran to take the appropriate measures following what he said was the "catastrophe" of it inadvertently shooting down the Ukrainian plane near the Iranian capital. Heiko Maas told Funke Mediengruppe media: "Its important that Iran has brought clarity. Now it should take the appropriate measures in the further investigation of this horrible catastrophe so that something like this cannot happen again." Maas said, "In these hours, our thoughts are with the victims and their families in all the affected nations." Furthermore, British Prime Minister Boris Johnson said on Saturday that the accidental shooting down of the Ukrainian passenger jet reinforced the importance of de-escalating tensions in the Middle East. "Iran´s admission that Ukraine International Airlines Flight 752 was shot down by mistake by its own armed forces is an important first step," Johnson said in a statement. Four British nationals were among the killed in Wednesdays crash. "We now need a comprehensive, transparent, and independent international investigation and the repatriation of those who died. The UK will work closely with Canada, Ukraine, and our other international partners affected by this accident to ensure this happens," Johnson said. "We can all see very clearly that further conflict will only lead to more loss and tragedy. It is vital that all leaders now pursue a diplomatic way forward."