A Boeing 737-800 of the Ukraine International airline parks at the airport in Duesseldorf, Germany. File photo/AFP

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.

All 176 aboard crashed Ukrainian jet dead

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 jet was on fire while still aloft.

The three-year-old jet, 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).

A Canadian security source told Reuters there was evidence one of the jet's engines had overheated.

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.

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.

Reuters