A missile strike or terrorism are now being investigated as possible causes of the Ukrainian Airlines plane crash in Iran that killed all 176 people on-board.

US officials are saying they are confident Ukrainian Flight 752 was shot down by Iran after satellite detected infra-red blips of two missile launches, probably SA-15s, followed shortly by another explosion, Newsweek reports. The publication cites its sources as a Pentagon official, a senior US intelligence official and an Iraqi intelligence official. The American officials reportedly told Newsweek that the Pentagon's assessment is that the incident was accidental.

Donald Trump said he suspected that the recent plane crash in Iran was not due to mechanical issues, indicating that "somebody could have made a mistake on the other side".

Asked during a White House event what he thought happened to the plane, the US president said: "Well, I have my suspicions".

"I don't want to say that because other people have their suspicions," Trump said. "Somebody could have made a mistake on the other side … not our system. It has nothing to do with us.

"It was flying in a pretty rough neighbourhood. They could've made a mistake. Some people say it was mechanical. I personally don't think that's even a question."

Asked if he thought it was downed by accident, Trump said, "I don't know. I really don't know … that's up to them. At some point they'll release the black box."

"Ideally they'd give it to Boeing," he said, but said giving it to France or "some other country" would be fine, too.

"Something very terrible happened, very devastating," he concluded.

Earlier today, Oleksiy Danilov, secretary of the National Security and Defence Council of Ukraine, said that investigators were following up on unconfirmed reports that fragments of a Russian-made Tor surface-to-air missile — a system used by Iran — had been found near where the plane came down. He didn't rule out a range of other possibilities for the disaster.

Ukraine was negotiating with Iran to allow the investigators to search the crash site near Tehran for possible rocket fragments, he told Censor.net, a Ukrainian news outlet.

The possibilities of a terrorist act, a collision with an airborne object such as a drone, and an engine explosion were also being examined as potential causes of the crash, Danilov said on his Facebook page.

The plane crashed on the outskirts of Tehran during a takeoff attempt on Wednesday hours after Iran launched its missile attack on US forces, scattering flaming debris and passengers' belongings across farmland and killing everyone on board.

The Iranian military disputed any suggestion the plane had been blown out of the sky by a missile, and Iranian aviation authorities said they suspected a mechanical problem brought down the three-year-old Boeing 737. Ukrainian officials initially agreed but later backed away and declined to offer a cause, citing an ongoing investigation.

Ukraine's President Volodymyr Zelenskiy cut short a visit to Oman to return to Kiev and said a team of Ukrainian experts would go to Tehran to help investigate the crash.

"Our priority is to find the truth and everyone responsible for the tragedy," he wrote in a Facebook statement.

Major world airlines on Wednesday rerouted flights crossing the Middle East to avoid danger amid escalating tensions between the US and Iran, and the US Federal Aviation Administration barred American flights from certain Persian Gulf airspace, warning of the "potential for miscalculation or misidentification" of civilian aircraft.

The plane had been delayed from taking off from Tehran's Imam Khomeini International Airport by almost an hour. It never made it above 8000 feet, crashing just minutes after takeoff, according to data from the flight-tracking website FlightRadar24.

Qassem Biniaz, a spokesman for Iran's Road and Transportation Ministry, said it appeared a fire erupted in one of its engines and the pilot lost control of the plane, according to the state-run IRNA news agency. The news report did not explain how Iranian authorities knew that.

The pilot apparently couldn't communicate with air-traffic controllers in Tehran in the last moments of the flight, said Hassan Razaeifar, the head of the air crash investigation committee. He did not elaborate.

Ukraine International Airlines President Yevhen Dykhne, said the aircraft "was one of the best planes we had, with an amazing, reliable crew." In a statement, the airline went further, saying: "Given the crew's experience, error probability is minimal. We do not even consider such a chance." General Abolfazl Shekarchi, spokesman for the Iranian armed forces, was quoted by the semi-official Fars news agency as denying the plane has been brought down by a missile.

"The rumours about the plane are completely false and no military or political expert has confirmed it," he said. He said the rumours were "psychological warfare" by the government's opponents.

Authorities said they found the plane's so-called black boxes, which record cockpit conversations and instrument data. But it was not immediately clear how much access to the information the Iranians would allow.

Aviation experts were sceptical about Iran's initial claim that the plane was brought down by a mechanical problem.

"I don't see how they would have known that so quickly," said John Hansman, an aeronautics professor at MIT. "They hadn't had time to look at the flight data recorder. They probably hadn't had time to investigate the physical wreckage of the engines. How do you know it was a mechanical issue versus a surface-to-air missile that went in the engine?" Many planes have systems that send huge amounts of technical data, including potential problems with the engines or other key systems, to the airline and the manufacturer. But it was unclear whether Ukraine International had paid to download that information automatically during flights, or how much data from such a short flight would tell.

A Boeing spokesman declined to say whether the company obtained any information about the jet during its ill-fated flight.

In 1988, a US navy cruiser mistakenly shot down an Iranian passenger jet over the Persian Gulf, killing 290 people. Earlier this week, Iranian President Hassan Rouhani referred to that episode in responding to Trump's threat to attack 52 targets in Iran.

"Those who refer to the number 52 should also remember the number 290," Rouhani tweeted. "Never threaten the Iranian nation."

Din Mohammad Qassemi said he had been watching the news about the Iranian missile attack on US troops in Iraq in revenge for the killing of General Qassem Soleimani when he heard the crash.

"I heard a massive explosion and all the houses started to shake. There was fire everywhere," he said. "At first I thought (the Americans) have hit here with missiles and went in the basement as a shelter. After a while, I went out and saw a plane has crashed over there. Body parts were lying around everywhere." The crash left a wide field of debris scattered across farmland, the dead lying among pieces of the shattered aircraft. Their possessions, including a child's cartoon-covered electric toothbrush, a stuffed animal, luggage and electronics, stretched everywhere.

Rescuers in masks shouted over the noise of hovering helicopters. They quickly realised there would be no survivors.

This was the first fatal crash involving Ukraine International Airlines, which began flying in 1992, according to the Aviation Safety Network, which tracks accidents.