The Kiev-bound Ukraine International Airlines plane was accidentally shot down shortly after takeoff from Tehran's Imam Khomeini International Airport on January 8.

Iran has come under mounting international pressure to carry out a full and transparent investigation into the air disaster.

"Investigators... discovered that two Tor-M1 missiles... were fired at the aircraft," Iran's Civil Aviation Organisation said in a preliminary report posted on its website late Monday.

It said an investigation was ongoing to assess the bearing their impact had on the accident.

The statement confirms a report in The New York Times which included video footage appearing to show two projectiles being fired at the airliner.

The Tor-M1 is a short-range surface-to-air missile developed by the former Soviet Union that is designed to target aircraft or cruise missiles.

Iran had for days denied Western claims based on US intelligence reports that the Boeing 737 operating Flight PS752 had been shot down.

It came clean on January 11, with the Revolutionary Guards' aerospace commander Brigadier General Amirali Hajizadeh accepting full responsibility.

But he said the missile operator who opened fire had been acting independently.

Black boxes

The deadly blunder triggered days of student-led protests mainly in the Iranian capital.

Supreme leader Ayatollah Ali Khamenei said on Friday that the demonstrations were unrepresentative of the Iranian people and accused the country's enemies of exploiting the air disaster for propaganda purposes.

In its report, the Civil Aviation Organisation said it was "impossible" for it to read the flight data and cockpit voice recorders -- commonly known as black boxes -- because they are so advanced.

But it suggests Iran wants to keep them for the time being.

"If devices are provided, the information (on the black boxes) can be restored and retrieved in a short period of time," it said.

The Civil Aviation Organisation said it had asked its French and US counterparts, the BEA and NTSB respectively, to provide a list of the equipment required to read the black boxes.

It said it had also sought the transfer of the required equipment, but added that neither the BEA nor NTSB had "so far responded positively" to such a transfer.

It said it had acquired the list nonetheless, without saying how, and hinted that it would use it to buy the equipment itself.

_AFP