Tehran,  Iran on Saturday said it unintentionally shot down the Ukrainian airliner that crashed this week killing 176 people, calling it an “unforgivable mistake”.

The Ukraine International Airlines plane came down on Wednesday shortly after Iran launched missiles at bases hosting American forces in Iraq in response to the killing of Qasem Soleimani, one of Iran’s top generals, in a US drone strike.

“The Islamic Republic of Iran deeply regrets this disastrous mistake,” Iranian President Hassan Rouhani tweeted.

“Armed Forces’ internal investigation has concluded that regrettably missiles fired due to human error caused the horrific crash of the Ukrainian plane & death of 176 innocent people,” he added.

“Investigations continue to identify & prosecute this great tragedy & unforgivable mistake.”

Earlier, Iran’s official IRNA news agency had published a statement from the military saying the Boeing 737 was mistaken for a “hostile plane” at a time when enemy threats were at the highest level.

The admission came a day after Iran’s civil aviation chief denied claims that the plane had been shot down, as international pressure mounted on Tehran to conduct a credible investigation after several Western governments blamed a missile strike.

The disaster came as tensions soared in the region after the Soleimani killing, and fears grew of an all-out war between the United States and Iran.

Washington has said the Soleimani strike was carried out to prevent “imminent”, large-scale attacks on US embassies. Iran had vowed “severe revenge” for Soleimani before launching missiles at the bases in Iraq.

“Human error at time of crisis caused by US adventurism led to disaster,” Iranian Foreign Minister Javad Zarif tweeted.

“Our profound regrets, apologies and condolences to our people, to the families of all victims, and to other affected nations.” Iran has invited the United States, Ukraine, Canada and others to join the crash investigation.

The majority of passengers on UIA Flight PS752 which had just taken off from Tehran were Iranian-Canadian dual nationals but also included Ukrainians, Afghans, Britons and Swedes.

It was Iran’s worst civil aviation disaster since the US military shot down an Iran Air plane over the Gulf by mistake in July 1988, killing all 290 people on board.

Video footage of the UIA 737, which the New York Times said it had verified, emerged and appeared to show the moment the airliner was hit.

A fast-moving object is seen rising at an angle into the sky before a bright flash appears, which dims and then continues moving forward. Several seconds later, an explosion is heard and the sky lights up.

Many airlines from around the world cancelled flights to and Iran in the wake of the crash, or rerouted flights away from Iranian airspace.

Nations around the world have called for restraint and de-escalation, and fears of a full-blown conflict have subsided after US President Donald Trump said on Wednesday that Iran appeared to be standing down after targeting the US bases in Iraq.