The Boeing 737-800 went down just minutes after taking off from Tehran's airport, leaving no survivors.

An initial probe found the aircraft experienced a problem as it was leaving the airport zone, and was "on fire".

The crash came just hours after Iran carried out missile strikes on two air bases housing US forces in Iraq.

However there is no evidence the two events are linked.

On Thursday, Oleksiy Danylov, the secretary of Ukraine's security council, said investigators wanted to search for possible debris from a missile at the site of the crash. Iran is known to have Russian missile defence systems.

Amid high tensions - worsened by the US killing of top Iranian general Qasem Soleimani on 3 January - Iran has said it will not hand over the recovered black box flight recorders to Boeing or to the US.

Under global aviation rules Iran has the right to lead the investigation, but manufacturers are typically involved.

Iran's Civil Aviation Organisation (CAOI) chief Ali Abedzadeh said: "The plane, which was initially headed west to leave the airport zone, turned right following a problem and was headed back to the airport at the moment of the crash."

Mr Abedzadeh added that witnesses saw the plane "on fire" before the crash, and that pilots hadn't made any distress calls before trying to return to Imam Khomeini airport.

He said the initial findings had been sent to Ukraine and the US, where Boeing is headquartered. Sweden and Canada had also been sent the findings, as their nationals were on board, he added.

Mr Danylov said that the Ukrainian security council was examining various possible causes, including an anti-aircraft missile strike, a mid-air collision, an engine explosion or an explosion inside the plane carried out by a terrorist.

The investigation would include experts who worked on the probe into the 2014 downing of Malaysia Airlines flight MH17 in eastern Ukraine, Mr Danylov added.

Ukraine has declared 9 January a day of national mourning.

Normally, the US National Transportation Safety Board would have a role to play in any international investigations involving US-made Boeings. But the board must act with permission and in accordance with legislation of the foreign country concerned.

In comments published by Iran's conservative Mehr news agency, Mr Abedzadeh, said: "We will not give the black box to the manufacturer and the Americans."

"This accident will be investigated by Iran's aviation organisation but the Ukrainians can also be present," he added.

Mr Abedzadeh said it was not yet clear which country would analyse the black boxes - a cockpit voice recorder and flight data recorder.

Ukrainian President Volodymyr Zelensky said in a televised statement that "a thorough and independent investigation will be conducted in accordance with international law", and that he would speak to Iranian leaders to step up cooperation in investigating the crash.

Boeing said it was "ready to assist in any way needed", while Canadian Prime Minister Justin Trudeau said his country expected to have a role in the investigation and had offered technical assistance.