Kiev is investigating whether Iran accidentally shot down a civilian airliner near Tehran using a Russian-made missile, its most senior security official has said.

Oleksiy Danilov, secretary of Ukraine’s Security Council, said officials were in Iran with a brief to look for any possible missile debris or evidence of a rocket strike.

“A rocket strike, possibly a Tor missile system, is among the main working theories, since there is information on the internet about elements of a missile being found near the site of the crash,” he said.

The British government said it was looking into “concerning” reports that the jet, which crashed on Wednesday, killing 176 people, may have been shot down. Briefing journalists following a call between prime minister Boris Johnson and Ukrainian president Volodymyr Zelensky, a spokesman called for a “credible and transparent investigation into what happened”.

Over the past day, at least two images of what appeared to be missile debris from Russian-made Tor missiles have appeared on social media. The photographs had not previously been uploaded to the internet, but attempts to geolocate them to the crash site have so far failed, and they may be unconnected.

Iran plane crash: Ukraine Boeing 737 comes down near Tehran Show all 18 1 /18 Iran plane crash: Ukraine Boeing 737 comes down near Tehran Iran plane crash: Ukraine Boeing 737 comes down near Tehran People stand near the wreckage after a Ukrainian plane carrying 176 passengers crashed near Imam Khomeini airport in Tehran ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran People and rescue teams are pictured amid bodies and debris All 176 people on board a Ukrainian passenger plane were killed when it crashed shortly after taking off, Iranian state media reported ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran One of the engines State news agency IRNA said 167 passengers and nine crew members were on board the aircraft operated by Ukraine International Airlines Iran Press via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue teams work at the scene AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran Passengers' belongings West Asia News Agency via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Members of the International Red Crescent collect bodies of victims EPA Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue teams work amidst debris AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran A relative of a victim reacts at Boryspil International Airport, outside Kiev Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers search the scene where a Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane with more than 170 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran TOPSHOT - People and rescue teams are pictured amid the wreckage after a Ukrainian plane carrying 176 passengers crashed near Imam Khomeini airport in the Iranian capital Tehran early in the morning on January 8, 2020, killing everyone on board. - The Boeing 737 had left Tehran's international airport bound for Kiev, semi-official news agency ISNA said, adding that 10 ambulances were sent to the crash site. (Photo by Rouhollah VAHDATI / ISNA / AFP) (Photo by ROUHOLLAH VAHDATI/ISNA/AFP via Getty Images) ROUHOLLAH VAHDATI ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers carry the body of a victim of an Ukrainian plane crash in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Part of the wreckage Iran Press via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Debris of a plane belonging to Ukraine International Airlines, that crashed after taking off from Iran's Imam Khomeini airport, is seen on the outskirts of Tehran, Iran January 8, 2020. Nazanin Tabatabaee/WANA (West Asia News Agency) via REUTERS ATTENTION EDITORS - THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY WANA NEWS AGENCY West Asia News Agency via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers search the scene where an Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue team work at the scene where an Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran CORRECTS YEAR - Debris is seen from a plane crash on the outskirts of Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying at least 170 people crashed on Wednesday shortly after takeoff from Tehranâ€™s main airport, killing all onboard, state TV reported. (AP Photos/Mohammad Nasiri) Mohammad Nasiri AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Officials inspect the wreckage EPA Iran plane crash: Ukraine Boeing 737 comes down near Tehran Debris is seen from an Ukrainian plane which crashed as rescue workers search the scene in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP

Mr Danilov said Ukraine was working on four theories. Three were concerned with an explosion of some sort: an explosion from a Russian-made missile strike, a bomb on board or a catastrophic explosion of the engine. The fourth possible cause was a flying object or drone colliding with the plane, he said.

Ukrainian investigators may have some difficulty in gathering that evidence. According to the country’s media reports, debris from the crash site has already been moved — raising doubts about the prospect of an independent investigation.

On Thursday, Yuri Butusov, a prominent journalist with strong links to Ukraine’s security services, published what he claimed to be the preliminary conclusions of an investigator working at the crash site. The source is quoted as saying there was no evidence of a fire inside the engines, instead suggesting the blaze originated inside the cabin for an as yet unknown reason.

“The absence of communications with air-traffic control and absence of fire means you can’t exclude the possibility of a terrorist attack, drone collision or missile strike,” the source is quoted as saying.

Flight data suggested Ukraine International Flight 752 took off normally from Tehran’s Imam Khomeini International Airport at 6.12am on Wednesday. Serious issues emerged two minutes later, about 8,000ft up, and the plane stopped transmitting data.

Footage released on the internet then shows the liner, a Boeing 737, engulfed in flames before plunging to the ground in a fireball and explosion. All 176 of those on board were killed, including nine crew.

Coming only a few hours after an Iranian attack on US bases in Iraq, the area was on high alert. American aviation authorities banned US airlines from flying in the region, lest a civilian plane be mistaken for a military one. Iranian authorities, however, did not introduce their own ban.

Aviation experts say a catastrophic event in an engine leading to the destruction of the plane itself would be an extraordinary occurrence. Modern civil liners such as the Boeing 737 are designed to fly safely on one engine.

None the less, early theories about the cause of the crash focused on engine failure. On Wednesday evening, a Reuters report, citing five intelligence officers, said the western intelligence community was minded to believe technical malfunction was most likely. Ukraine’s embassy in Iran initially took this view, ruling out the possibility of a missile attack, before hastily removing the statement.

Iranian officials almost immediately ruled out all causes that were not mechanical — itself an unusual step so early on in the investigation.

On Thursday, Iran’s civil aviation authorities issued a report that suggested the plane was hit by a “sudden emergency”. The crew did not declare an emergency, it noted, but were attempting to turn back towards the airport at the time of the crash.

Ukraine’s security chief Mr Danilov said that 45 Ukrainian investigators were already working in Tehran. Their initial priority would be to look for evidence of a rocket attack and to verify the images online.

Eliot Higgins, the founder of Bellingcat, the digital investigation group famous for its investigation into the downing of MH17 over eastern Ukraine in 2014, confirmed that the photograph of Tor missile debris were new images as far as the internet was concerned. But on Twitter he suggested independent verification may prove elusive.

“There’s other examples of this type of debris documented in other conflicts, so there’s no way to know this is in Iran,” he said.

The Tor is a highly mobile Russian-made system used for low-altitude anti-aircraft tasks. Tehran took 29 from Moscow in 2007 in a massive arms deal.

So far, the investigation has been marred by confusion and controversy. Iran has signalled that it will not be sending the black boxes to Boeing or America. According to Vadim Lukashevich, an independent Russian aviation expert, Iran does not possess the expertise to analyse the flight recorders independently.