London: Ukraine says it is investigating whether a packed passenger plane that crashed shortly after take-off in Iran was shot down by a missile or hit by a bomb, casting further doubt over previous claims the aircraft experienced mechanical difficulties.

The fiery Ukraine International Airlines crash killed 176 passengers and crew on Wednesday - just as the region was gripped by a heightened state of alert over Iran's decision to launch ballistic missiles on military bases in Iraq housing US troops.

Officials from Iran and Ukraine quickly downplayed fears that the disaster - which killed everyone on board - was connected to the tensions, but Kiev later withdrew its assurances that mechanical error was to blame.

The secretary of Ukraine's national security council, Oleksiy Danylov, has now told Ukrainian media that investigators want to search for the possible debris of a Russian missile at the crash site near Tehran's main airport, but offered no evidence to support that line of inquiry other than citing "information" on the internet.