Iran said on Saturday it unintentionally shot down a Ukrainian passenger plane killing 176 people on board due to human error, after initially denying it brought down the plane in the tense aftermath of Iranian missile strikes on U.S. targets in Iraq.

Wednesday's crash heightened international pressure on Iran after months of friction with the United States and tit-for-tat military strikes. Washington killed an Iranian general last week in Iraq, prompting Tehran to fire at U.S. targets.

The United States and Canada, which had 57 citizens on board, had blamed an Iranian action for bringing down the aircraft. Ottawa had told Iran that "the world is watching."

On Twitter, Iranian Foreign Minister Mohammad Javad Zarif said the armed force's investigation showed the downing of the Boeing 737-800 was the result of "human error at time of crisis caused by U.S. adventurism (that) led to disaster."

An Iranian military statement, announcing that a missile had struck the plane and expressing condolences to the victims, said the plane had flown close to a sensitive military site belonging to the elite Revolutionary Guards.

It said responsible parties would be referred to a judicial department within the military and held accountable.

Mobile phone footage posted and circulated by ordinary Iranians on Twitter after the crash has indicated that it came down in a ball of flames.

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 that the process could take one to two months.

Tehran said it could ask Russia, Canada, France or Ukraine for help in an effort that it said could take one or two years.

Many of the victims were Iranian with dual nationality.

Iran initially dismissed the accusations that a missile was to blame for the crash, calling such suggestions "psychological warfare".

Grief-stricken Iranians and others have posted images related to the crash. One showed a child's red shoe in the dirt. Another was a selfie of a mother and daughter in their seats, sent to a loved one just before takeoff.

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

Ukraine had been looking at various possible causes of the crash, including an attack by a Russian-made missile, a collision, an engine explosion or terrorism.