Ukranian investigators have identified four possible reasons for the Iran plane crash, which claimed 176 lives.
Oleksiy Danylov, secretary of Ukraine's National Security and Defence Council, revealed that a missile attack, a collision, an engine explosion or terrorism could have led to the tragedy.
It comes after an initial report by Iranian investigators said the plane, a Boeing 737-800, had been on fire immediately before it crashed.
Ukraine is looking at various possible causes of the crash, including a possible missile attack, a collision, an engine explosion or terrorism, he wrote in a Facebook post.
The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, leading some to speculate that the plane may have been hit.
Photos circulating on social media allegedly show a fragment of a rocket, and posts claim the debris was found near the site where Ukrainian International Airlines flight 752 went down.
However an initial assessment by Western intelligence agencies was that the plane had suffered a technical malfunction, five security sources told Reuters.
Danylov said Ukrainian investigators in Iran wanted to search the crash site for possible debris of a Russian missile after seeing reports about its possible existence on the internet.
He referred to an unverified image being circulated on Iranian social media purportedly showing the debris of a Russian-made Tor-M1 surface-to-air missile of the kind used by the Iranian military.
"Our (investigative) commission is talking to the Iranian authorities about visiting the crash site and is determined to search for fragments of a Russian Tor air defence missile about which there was information on the internet," Danylov separately told Ukrainian news site Censor.net.
He said Ukraine would draw on expertise learnt from carrying out its own investigation into the 2014 shooting down of
Malaysia Airlines flight MH-17 over eastern Ukraine, an incident that killed all 298 people on board.
Bellingcat, an investigative website that did groundbreaking work on the July 2014 downing of a Malaysia Airlines flight over eastern Ukraine, questioned whether the image was what it claimed to be.
It tweeted: "The problem with this photo of the remains of an AA missile is it's been taken at an angle where it'll be next to impossible to geolocate, so unless another image appears which can be geolocated it won't be possible to verify it's in Iran."
Bellingcat founder Eliot Higgins added: “Until the location of this debris is confirmed, which is next to impossible with this image, I would treat it with caution.
"There's other examples of this type of debris documented in other conflicts, so there's no way to know this is in Iran."
Ashkan Monfared, who posted the image on Twitter, where he described himself as an Iranian activist living in the US, wrote: "The photo was taken by an amateur who had no knowledge of the significance of the story and did not know what he had found. Hope to have more photos. I texted her and didn't receive a reply yet."
The crash site is less than two miles from an Iranian military base.
