London: Ukraine has withdrawn its previous assurances that mechanical failure was to blame for a plane crash over Iran that killed 176 passengers and crew, with experts warning a potential "catastrophic" event may have caused the airliner to plummet to the ground.

The crash of the Ukraine International Airlines flight minutes after takeoff from Tehran's airport occurred just hours after the regime launched a ballistic missile attack on military bases housing American troops in neighbouring Iraq.

Officials from Iran and Ukraine quickly downplayed fears the crash - which killed everyone on board - was connected to the tensions in the region.

Both countries initially blamed mechanical error amid claims from Iranian officials that one of the airliner's engines had caught fire, causing the pilot to lose control and crash into a field.