DUBAI (REUTERS) - Analysis from the black boxes of a downed Ukrainian passenger plane shows it was hit by two missiles 25 seconds apart and that passengers were still alive for some time after the impact of the first blast, Iran said on Sunday (Aug 23).

The announcement by the head of Iran's Civil Aviation Organisation marks the first official report on the contents of the cockpit voice and data recordings, which were sent to France for reading in July.

Teheran has said it accidentally shot down the Ukraine airliner in January, at a time of extreme tensions with the United States. All 176 people aboard the plane were killed.

"Nineteen seconds after the first missile hit the plane, the voices of pilots inside the cockpit, indicated that the passengers were alive... 25 seconds later, the second missile hit the plane," Mr Touraj Dehghani-Zanganeh was cited as saying by state television.

"Therefore, no analysis of the performance and effects of the second missile was obtained from the aircraft's black box."

The aircraft's flight crew - two pilots and an instructor also travelling in the cockpit - tried to keep control of the plane until the last moment, Mr Zanganeh said.

Iran has been in talks with Ukraine, Canada and other nations that had citizens aboard the downed plane, and who have demanded a thorough investigation into the incident.

Iran's Revolutionary Guards shot down the Ukraine International Airlines flight with a ground-to-air missile on Jan 8, just after the plane took off from Teheran, in what Teheran later acknowledged as a "disastrous mistake" by forces on high alert during a confrontation with the United States.

Iranian and Ukrainian officials have held talks on the compensation to families of the victims. Another round of talks is set for October.

Iran's investigation is being carried out under United Nations aviation rules calling for probes aimed solely at preventing future accidents, separately from any judicial process. But the probe has been swept up in regional and domestic tensions.

"The data analysis from the black boxes should not be politicised," Mr Zanganeh said.

Over 20 people have been sentenced to jail terms up to 20 years in Iran for participating in peaceful protests against the downing of the plane.

Some Iranians took to Twitter on Sunday to show their anger, with at least one user tweeting "They were alive for 19 seconds ... a tragedy".