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By Aresu Eqbali in Tehran and Rory Jones in Dubai 

Iran said a Ukrainian airliner that crashed outside of an airport in Tehran, killing all 176 people on board, was shot down unintentionally by its armed forces.

Iranian officials had initially denied claims by Western officials that the Boeing Co. 737-800 jet was downed by a missile system fired by Iran, possibly by mistake.

The single-aisle jet crashed Wednesday four hours after Iran had launched military strikes against the U.S. in Iraq in retaliation for the killing of a top general, and the Middle Eastern state was on high alert for a potential U.S. response.

The Ukraine International Airlines plane approached a sensitive military base operated by the Islamic Revolutionary Guard Corps and was downed due to "human error," Iran's general staff of the armed forces said in a statement on state television early Saturday local time.

The armed forces detected greater U.S. air activity and U.S. warplanes on its radar after the Iranian strikes on American forces in Iraq, the statement said.

Iranian President Hassan Rouhani tweeted that the downing was a "great tragedy & unforgivable mistake."

Iranian Foreign Minister Javad Zarif said the error was caused by "U.S. adventurism" in the Middle East. "Our profound regrets, apologies and condolences to our people, to the families of all victims, and to other affected nations," he tweeted.

The airline and a spokeswoman for Ukrainian President Volodymyr Zelensky didn't immediately respond to a request for comment.

Since the Iranian strikes, President Trump has said the U.S. didn't plan to respond to Iran and that Iranian forces appeared to be standing down from further conflict with American forces.

The downing of the jet, which killed a number of Iranians, is likely to place further pressure on Iran's leadership, already challenged by U.S. sanctions that have prompted an economic crisis and widespread public protest.

Until Saturday, Iranian officials had repeatedly denied that the jet was shot down by its armed forces, despite claims by U.S., U.K. and Canadian officials that it crashed after being hit by missiles.

Iranian investigators had said the plane likely crashed due to a technical error and the pilot was attempting to return to Tehran's main international airport when it hit the ground.

On Friday, Tehran gave Ukrainian investigators access to the so-called black-box data and cockpit voice recorders, and Iranian investigators said they would be willing to hand them over to other countries as well for analysis.

Ukraine's foreign minister, Vadym Prystaiko, told reporters Friday that so far Iran has granted full access for Ukraine to conduct its investigation. A team of 45 Ukrainian experts and officials is on the ground in Tehran, Ukrainian officials said.

Iranian officials also have invited investigators from Boeing, the U.S., Ukraine, France and Canada to probe the causes of Wednesday's crash, including a number of Canadians.

The National Transportation Safety Board, the U.S. entity tasked with transport-accident investigations, said it is monitoring the situation and evaluating its level of participation.

Boeing has started preparing the necessary paperwork and has been in touch with both the State and Commerce departments, The Wall Street Journal reported Thursday.

In the normal course of an investigation, some level of participation would also be expected from the Federal Aviation Administration and General Electric Co., which made the plane's engines with a French joint-venture partner.

Canada -- which said late Friday it lost 57 citizens in the crash, down from its earlier statement of 63 -- has vowed to work with international partners to ensure it is thoroughly investigated.

The Iranian investigation team has said it would first seek to analyze the black boxes in Iran. They could also be examined in Russia, Ukraine, France or Canada, four countries that had expressed a readiness to help, Iranian investigators said.

U.S. industry and government officials say the Iranians have the ability to conduct routine downloads of black-box recorders, but are skeptical they have the technical expertise and experience to extract data from damaged devices.

The crash comes after Iran has undergone a convulsive few months. At least 59 people died this week in a stampede at a funeral march for Maj. Gen. Qassem Soleimani, the military commander killed by a U.S. strike.

In November, hundreds of demonstrators were killed by security forces when they rose up across the country in protest against government austerity measures, prompted partly by American sanctions. The security response was the deadliest in decades.

The Ukrainian jet isn't the first time tensions between the U.S. and Iran have led to an aviation tragedy. In 1988, the U.S. Navy's Vincennes warship downed Iran Air Flight 655 with an air-defense missile.

U.S. officials said they mistook the Airbus SE A300 airliner for a warplane they feared would attack the ship. All 290 people, mostly Iranian, on the plane died, prompting an outpouring of national grief in Iran.

Georgi Kantchev in Kyiv contributed to this article.

Write to Rory Jones at rory.jones@wsj.com

(END) Dow Jones Newswires

January 11, 2020 00:47 ET (05:47 GMT)

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