(Adds details, background)
DUBAI, Jan 9 (Reuters) - A Ukrainian airliner was on fire immediately before it crashed southwest of Tehran killing all 176 on board, according to an initial report by Iranian investigators.
The Ukrainian International Airlines Boeing 737-800, flying to Kiev and carrying mostly Iranians and Iranian-Canadians, crashed shortly after taking off on Wednesday from Tehran's Imam Khomeini airport.
The report by Iran's civil aviation organisation cited witnesses on the ground and in a passing aircraft flying at high altitude as saying the plane was on fire while still aloft.
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.
The technical problem was not specified in the report, which also said that there was no radio communication from the pilot and that the aircraft disappeared from radar at 8,000 feet (2,440 m).
It is so far not clear if any technical issue could be related to a mechanical fault or defective part.
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. Issuing an initial report within 24 hours is rare and it can take months to fully determine the cause.
A Canadian security source told Reuters there was evidence one of the engines had overheated.
The crash happened hours after Iran launched missile attacks on U.S.-led forces in Iraq, leading some to speculate that the plane may have been hit.
The initial assessment of Western intelligence agencies was that the plane had suffered a technical malfunction and had not been brought down by a missile, five security sources - three Americans, one European and the Canadian - who asked not to be named, told Reuters.
In Kiev, Ukraine's President Volodymyr Zelenskiy said the government was considering several possible causes of the plane crash.
In a television statement, Zelenskiy asked people to refrain from speculation, conspiracy theories and hasty evaluations regarding the crash. He also declared Thursday a day of national mourning.
He said that he would speak by telephone with the Iranian president to step up cooperation into finding out the reason for the crash.
Countries recognised as participants in the investigation should nominate who they wish to be involved, the Iranian report said, while saying Ukrainian experts would take part.
As the country where the plane was designed and built, the United States is usually allowed to be accredited under a UN-administered convention but neither side has said whether US investigators will be dispatched to Iran.
Iran's aviation body could not be reached for comment to clarify their position.
Tensions between Washington and Tehran have risen with the United States' killing of a top Iranian general on Friday and Tehran retaliated with a missile strike on U.S. targets in Iraq.
The Ukrainian airliner took off from the Tehran airport at 6:12 a.m. 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, were strewn across a field where the plane crashed on Wednesday. Rescue workers in face masks laid out scores of body bags.
Onboard were 146 Iranians, 10 Afghans, 11 Ukrainians, five Canadians and four Swedes, the report said, but said some may have held citizenship of other countries.
Ukrainian authorities have said those on board included 82 Iranians, 63 Canadians, and 11 Ukrainians.
The Tehran-Toronto via Kiev route was a popular for Canadians of Iranian descent visiting Iran in the absence of direct flights. (Writing by Alexander Cornwell; Additional reporting by Pavel Polityuk, Editing by Angus MacSwan)