* Jet caught fire before crash - report
* Ukraine security official sets out four theories* Ukraine wants to search crash site for Russian missile debris
* National day of mourning in Ukraine
A Ukraine airliner that crashed in Iran, killing all 176 people aboard, was most likely brought down accidentally by Iranian air defenses, US officials said on Thursday, as President Donald Trump said he had a terrible feeling about the disaster.
One US official said US satellites had detected the launch of two missiles shortly before the plane crashed, followed by evidence of an explosion. Two officials said Washington believed the downing of the plane was accidental.
Speaking to reporters, Trump said the crash could have been a mistake, adding he had a terrible feeling about the downed airliner but offering no evidence.
"Somebody could have made a mistake," Trump told reporters at a the White House, adding that he had suspicions about the crash but giving no other details.
Ukraine meanwhile outlined four potential scenarios to explain the crash, including a missile strike and terrorism, as Iranian investigators said the plane was on fire before it fell to the ground.
Kiev said its investigators wanted to search the site of Wednesday's crash southwest of Tehran for possible debris of a Russian-made missile used by Iran's military. An initial report by Iran's civil aviation organisation said the plane had experienced an unspecified technical problem.
The Ukrainian International Airlines Boeing 737-800, flying to Kiev and carrying mostly Iranians and Iranian-Canadians, crashed shortly after taking off from Tehran's Imam Khomeini airport, killing all 176 people on board.
The Iranian report cited witnesses on the ground and in a passing aircraft flying at a high altitude as saying the plane was on fire while in the air.
It said the three-year-old airliner, which had its last scheduled maintenance on Monday, encountered a technical problem shortly after take-off and started to head toward a nearby airport before it crashed. The report said there was no radio communication from the pilot and that the aircraft disappeared from radar at 8,000 feet (2,440 m).
INITIAL REPORT
The disaster puts a renewed spotlight on Boeing, which faces a safety crisis over a different type of 737, though the plane that crashed in Iran does not have the feature thought to have caused crashes of the grounded 737 MAX.
The Iranian report referred to the crash as an accident.
Investigations into airliner crashes are complex, requiring regulators, experts and companies across several international jurisdictions to work together. It can take months to fully determine the cause and issuing an initial report within 24 hours is rare.
The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, leading some to speculate that the plane may have been hit.
Canadian Foreign Minister Francois-Philippe Champagne called his Iranian counterpart to stress the need for Canadian officials "to be quickly granted access to Iran to provide consular services, help with identification of the deceased and take part in the investigation of the crash", a Canadian statement said.
"Canada and Canadians have many questions which will need to be answered."
Britain wanted a transparent investigation, Prime Minister Boris Johnson's spokesman said on Thursday following a call between the British leader and Zelenskiy.
"The prime minister said that there needed to be a full credible and transparent investigation into what happened," the spokesman said.
Tensions between Washington and Tehran have risen with the United States' killing of a top Iranian general on Friday. Tehran retaliated with a missile strike on US targets in Iraq.
The Ukrainian airliner took off at 6:12am local time and was given permission to climb to 26,000 feet, the report said. It crashed six minutes later near the town of Sabashahr.
Bodies and body parts recovered from the site of the crash have been taken to the coroner's office for identification, the report said.
Smouldering debris, including shoes and clothes, was strewn across a field where the plane crashed. Rescue workers in face masks laid out scores of body bags.
