Flight recorders from the crashed Ukraine International Airlines Boeing 737-800 in Tehran have been retrieved, with their memory units, although both have signs of physical damage.

But there are no firm indications as to the circumstances of the 8 January event, including whether it might have been the result of hostile action or sabotage.

The Iranian Civil Aviation Organisation says both the flight-data and cockpit-voice recorders were recovered from the wreckage of the jet after it came down shortly after take-off from Tehran Imam Khomeini airport.

It states that the aircraft initially climbed to 8,000ft before radar contact was lost and the jet struck the ground. Ukraine International Airlines concurs with this figure, putting the altitude at about 2,400m (7,900ft).

“No radio transmissions from the pilot were received regarding unusual circumstances,” says the Civil Aviation Organisation’s chief, Ali Abedzadeh.

The aircraft’s trajectory was initially westwards, he says, but the jet started to turn right as the problem emerged, and appears to have been heading back to the airport.

Abedzadeh says that preliminary information – from eyewitnesses on the ground at crews at altitude – indicate the aircraft was on fire, before its impact with terrain and subsequent explosion.

The initial ground impact was in the vicinity of an amusement park, he says.

Officials in Ukraine and the USA have been notified along with governments in Sweden and Canada. Ukraine has been formally invited to participate in the accident inquiry.

Abedzadeh says 146 of the 167 passengers and nine crew members held Iranian passports.

He says the aircraft left the stand at 05:45 before its take-off at 06:13, and was initially cleared to Mehrabad control which cleared it to climb to 26,000ft. Contact was lost at 06:18 and the aircraft came down near the province of Saba Shahr.

The Civil Aviation Organisation says, in a preliminary document, that the aircraft started to taxi at 06:05.

No conclusions have been released on the circumstances of the crash. The aircraft was less than four years old, and powered by CFM International CFM56 engines. Preliminary weather data from Tehran indicates no adverse conditions at the time.

Ukraine International Airlines says the captain had logged 11,600h on 737s including 5,500 in command, while the first officer had 7,600h on type. The flight crew was augmented by an instructor pilot with 12,000h on 737s including 6,600h as captain.

It adds that Tehran is not a “simple” airport and that the carrier has, for several years, used Imam Khomeini as a training location for 737 crews to check pilot proficiency.

”Given the crew’s experience, error probability is minimal,” the airline claims. “We do not even consider such a chance.”