The plane crashed shortly after take off Wednesday, killing all 176 people on board, shortly after Iran fired a volley of missiles against military bases in Iraq housing US personnel.

"Several internal and international flights were flying at the same time in Iranian airspace at the same altitude of 8,000 feet (2,440 metres)," Iran's transport ministry said.

"This story of a missile striking a plane cannot be correct at all," it said in a statement.

"Such rumours make no sense," Ali Abedzadeh, head of Iran's civil aviation organisation and deputy transport minister, said in the statement.

Abedzadeh was reacting to rumours on social networks that the Boeing 737 was hit by a missile fired by Iran's Revolutionary Guards.

He said Iran and Ukraine were in the process of "downloading information" from black boxes retrieved from the crash site.

"But if more specialised work is required to extract and analyse the data, we can do it in France or another country," he added.

On Wednesday, Iran's Mehr news agency -- close to ultraconservatives -- quoted Abedzadeh as saying Iran "would not give the black boxes to the Americans".

But the minister's statement on Thursday rejected "rumours of Iran's resistance to delivering the black boxes... to the US".

Iran is not obliged to have the black boxes analysed in the US, but America is one of only a few countries -- including France and Germany -- capable of carrying out such work.

Iranian authorities say initial indications showed the plane had turned back after suffering a problem.

A team of Ukrainian experts flew in and joined the investigation on the ground Thursday.

Kiev said it was studying several scenarios, including an in-flight collision, a rocket strike, an engine explosion caused by a technical problem, and an onboard blast.

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