Iran called on Western governments to prove claims the Boeing Co. 737-800 passenger jet that crashed near Tehran on Wednesday was shot down, intensifying a standoff that could complicate an already difficult investigation fraught with geopolitical hurdles.

“If they are certain and have the courage, they should share any finding that has scientific and technical backing,” Ali Abedzadeh, head of Iran’s Civil Aviation Organization, said during a televised briefing Friday. Iran is “certain” that Ukraine International Airlines Flight 752 wasn’t shot down, he said, building on earlier government denials.

The comments come after the prime ministers of Canada, the U.K. and Australia said they had intelligence showing the plane was shot down by a missile. The crash, which killed all 176 people on board, comes at a time of heightened tensions between the U.S. and Iran, whose economy has been crippled by sanctions.

Still, Iran has said it invited Boeing and investigators from foreign countries including Canada to assist in the probe into the crash. More than a third of the passengers on the jet were from Canada, which said it is sending a team to help with the effort. The U.S. National Transportation Safety Board said it is monitoring the situation.

Meanwhile, the flight-data and cockpit-voice recorders will be examined Friday at Tehran’s Mehrabad International Airport and any claims of what happened should be considered speculation until the information is retrieved, Abedzadeh said. Hassan Rezaeifar, Iran’s head investigator, said in the same briefing that Iran is open to allowing Russia, Ukraine, France or Canada to take charge of extracting the data.

Intelligence from multiple sources, including Canada’s allies, “indicates that the plane was shot down by an Iranian surface-to-air missile,” Prime Minister Justin Trudeau told reporters in Ottawa on Thursday. “This may well have been unintentional.”

U.K. Prime Minister Boris Johnson issued a statement earlier Thursday saying there is evidence Flight 752 was shot down by an Iranian surface-to-air missile. “We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation,” Johnson said.

In Australia on Friday, Prime Minister Scott Morrison said in a radio interview that his country had intelligence that an Iranian missile had shot down the jetliner. In another radio interview, he said he had been briefed by Trudeau and described the episode as “a terrible accident.” In Ukraine, President Volodymyr Zelenskiy said while a missile hit hadn’t been ruled out, it also hadn’t been confirmed, “as of today.”

Whether accidental or intentional, a shoot-down would echo two other instances of surface-to-air missiles striking civilian jets. In 2014, pro-Russian rebels in Ukraine fired on and destroyed a Malaysia Airlines jetliner. In 1988, an Iranian airliner was felled by a U.S. cruiser after being mistaken for a hostile aircraft following a skirmish with Iranian boats.

Some airlines aren’t taking any chances. Deutsche Lufthansa AG tweeted Friday that it turned around a plane headed to Tehran.

President Donald Trump, speaking to reporters in Washington on Thursday, said, “I have my suspicions” about why the plane went down but he didn’t say what those suspicions are.

“It was flying in a pretty rough neighborhood,” Trump said. “Somebody could have made a mistake.”

Two surface-to-air missile launches were detected by a U.S. spy satellite from an Iranian battery near the airport minutes after the jet took off, followed by an explosion near the plane, said a person familiar with the investigation who spoke on the condition of anonymity.

The Russian-made SA-15 missile, also known as a Gauntlet or a Tor, is suspected of being involved. They are short-range weapons designed to attack planes, helicopters and other airborne targets.

The New York Times reported that it had obtained a video that appears to show an Iranian missile hitting a plane near the Tehran airport. The video shows a small explosion occurring when a missile struck the jet, which didn’t explode, according to the Times.

Boeing rose as much as 3.1 per cent and was up 1.5 per cent to US$336.34 Thursday in New York following reports indicating the plane was shot down. If the plane was brought down by a missile strike, it could rule out a mechanical failure that would affect other Boeing aircraft.

The Ukrainian president’s office has said 45 Ukraine experts were working in Iran on the investigation and there are “several versions” for the cause under consideration.

Iranian President Hassan Rouhani spoke by phone with his counterpart Zelenskiy on Thursday and agreed to form a task force involving their transport officials and foreign ministries, the semi-official Fars news agency reported, citing Rouhani’s deputy head of communications. Zelenskiy will discuss the crash with U.S. Secretary of State Michael Pompeo, the Ukrainian president said on Facebook.

The U.S. intelligence assessment is consistent with what some aviation accident experts have said. The apparent rapid spread of the fire combined with the sudden halt of radio transmissions from the plane after a normal climb aren’t consistent with previous crashes, said Jeffrey Guzzetti, the former head of accident investigations at the Federal Aviation Administration.

While Iranian officials initially said they suspected a problem with one of the plane’s engines, they retracted that in a preliminary report issued Thursday. The government also took the unusual step of setting up an investigative group to examine whether “any unlawful actions” initiated the fire on the plane, the preliminary report said.

Iran notified the International Civil Aviation Organization, an arm of the United Nations, about the crash, which should trigger involvement of other nations in the investigation, including the U.S., the agency said in a press release Thursday.

Under rules known as Annex 13, the nation in which a crash occurs usually is in charge of an investigation. Other nations are permitted to take part, such as the country in which the plane was made. Since Boeing manufactured the Ukrainian jet, the U.S. National Transportation Safety Board would have a right to participate.

It remains unclear whether NTSB will send a representative to Iran because U.S. law restricts travel to that country and the exchange of certain data. The agency said in a statement on Thursday night that it had “designated an accredited representative to the investigation of the crash.”

The U.S. Treasury has granted waivers for U.S. investigators to work in Iran in the past, but it has been a cumbersome process. Also, the NTSB has at times declined to send investigators to countries it deems unsafe.

--With assistance from Thomas Penny, Daryna Krasnolutska and Aliaksandr Kudrytski.