Flight 752, a Ukraine International Airlines passenger jet, crashed shortly after taking off from Tehran's main airport on January 8.

Iran admitted days later that its forces accidentally shot down the Kiev-bound Boeing 737-800 aircraft, killing all 176 people on board.

Tehran's air defences had been on high alert at the time in case the US retaliated against Iranian strikes hours earlier on American troops stationed in Iraq.

The head of Iran's civil aviation authority on Sunday revealed for the first time what was on the black boxes, which had been sent to France for analysis.

Touraj Dehghani Zanganeh said that the cockpit voice recorder registered a conversation between the pilot, co-pilot and an instructor between the two blasts.

"Up to 19 seconds after the first missile exploded in the vicinity of the aircraft, (they) noticed abnormal conditions and were in control of the aircraft until the last moment," he said, quoted by state television's website.

"The instructor indicates that the aircraft has an electronic problem and the auxiliary power has been activated," he said.

"The pilots were notified that both engines of the aircraft were on."

The black boxes stopped working 19 seconds after the first explosion, making it impossible to retrieve data on the impact of the second missile, he said.

Analysis on the "effect of the second missile cannot be obtained from the black boxes," said Zanganeh.

Iran, which has no means of decoding the black boxes, sent them to France for analysis in mid-July, nearly six months after the disaster.

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