Iran’s military announced early on Saturday that it had accidentally shot down a Ukrainian passenger jet, blaming human error because of what it called the plane’s sharp, unexpected turn toward a sensitive military base.

After days of tension since the jet crashed near Tehran on Wednesday, the same day that Iranian missiles struck American bases in Iraq, the admission was a stunning reversal. Iran initially maintained that mechanical issues had brought the Boeing airliner down, killing all 176 people aboard.

“The Islamic Republic of Iran deeply regrets this disastrous mistake,” President Hassan Rouhani said on Twitter soon after the military released its statement. He offered condolences to the victims’ families and said investigations were underway. The military said the person responsible would face legal consequences.

International pressure had been building on Iran to take responsibility. American and allied intelligence assessments had already concluded that Iranian missiles brought down the plane, Ukraine International Airlines Flight 752, most likely by accident, amid the heightened tensions between the US and Iran.

“The little credibility that the Islamic Republic had among its supporters suffered a major blow tonight,” said Rouzbeh MirEmbrahim, an independent Iran analyst in New York and a consultant with the UN. “This tragedy undermines the image Iran has cultivated as a military power and weakened it significantly both regionally and internationally.”

On social media, Iranians began expressing anger toward the military soon after the announcement, many of them using the term “harshest revenge”, which officials had repeatedly promised in the wake of the American drone strike that killed Major General Qassem Soleimani, a powerful Revolutionary Guards commander, last week.

“They were supposed to take their harsh revenge against America, not the people,” wrote Mojtaba Fathi, a journalist.

The Iranian military’s statement said the plane “took the flying posture and altitude of an enemy target” as it came close to an Islamic Revolutionary Guards Corps base. It said that “under these circumstances, because of human error”, the plane “came under fire”.

The military said it would undertake “major reform in operations of all armed forces” to make sure that such an error never happened again.

It said Revolutionary Guards officials had been ordered to appear on state media and give the public a full explanation.

In a statement of his own, Iran’s foreign minister, Mohammad Javad Zarif, tried to place some of the blame on the US, saying on Twitter that the disaster was “caused by US adventurism”.

The military’s statement said there had been information suggesting the US was “preparing to aerially target sensitive defence and key sites and multiple targets in our country, and this led to even more sensitive defence posture by our anti-aircraft units”. The state department had no immediate comment.

Suspicions that an Iranian missile had brought down the plane were raised immediately after the crash on Wednesday morning — just hours after Iran fired missiles at two bases in Iraq housing American forces.

The Iranians asked the National Transportation Safety Board to help with the investigation, and the state department granted waivers to allow the American agency to help.

A senior administration official said on Friday that he thought the Iranians wanted American investigators there to keep up the appearance that they did not know what had caused the crash.

The official said the Iranian military had poor command and control.