The Ukraine International Airlines (UIA) Boeing went down outside Tehran shortly after take-off on Wednesday, killing all 176 people on board.

"For the moment we are working on seven different versions of the event that took place in Iran," Oleksiy Danilov, the secretary of Ukraine's national security and defence council (RNBO), told AFP.

The council is tasked with coordinating the probe into the disaster, the first fatal crash involving Ukraine's biggest carrier UIA.

Danilov also said on Facebook that the leads being studied include both technical malfunctions and foul play, but told AFP that "there is no priority version" yet.

The leads under consideration include a collision with another airborne object, a rocket from Iran's missile defence system, an engine explosion caused by a technical problem, and an explosion on board the aircraft due to an "act of terror", he said on Facebook.

Danilov told AFP that for the moment there was no reason to believe that the airliner had been hit by a missile.

"We are very carefully studying all seven versions," he said.

Dozens of top Ukrainian experts flew to Tehran early Thursday and were currently working in the country, the council said.

The crash -- which killed mainly Iranians and Canadians -- came shortly after Tehran launched missiles at bases in Iraq housing American troops to avenge the US killing of a top Iranian general last week.

The airline said the Boeing 737 had been built in 2016 and checked only two days before the accident.

It said the pilots were well trained and that the plane's captain had more than 11,000 hours of flying experience, mostly on the Boeing 737.

Iranian authorities on Thursday said the plane turned around after taking off after suffering a "problem", disappearing from radars at an altitude of 2,400 metres (8,000 feet), but that the crew sent no message about an emergency.

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