A senior Ukrainian security official has said his country’s investigators will explore a range of possible reasons why one of its passenger jets crashed in Iran, including a drone collision, a terrorist bomb and a missile attack, but did not rule out a technical fault was to blame.

Oleksiy Danilov, the secretary of Ukraine’s national security council, cited unconfirmed reports circulating on social media that debris from a Russian-made missile had been found at the site, on the outskirts of Tehran, where the Ukraine International Airlines Boeing 737-800 crashed on Wednesday, killing all 176 passengers and staff onboard.

“Our commission is currently agreeing with the Iranian authorities to travel to the place of the crash, and plans to search for debris of a Russian surface-to-air Tor missile, according to information which was published on the internet,” he said in a Facebook post on Thursday.

Some members of the investigative team had been involved in the probe into the 2014 shooting down of Malaysia Airlines Flight 17 over eastern Ukraine by Russian-armed rebels, Danilov added. “We will use all our best practices from investigating the attack on MH17 to find out the truth in the case of the Ukrainian plane in Tehran,” he said.

Iran plane crash: burning Boeing was trying to turn back, say Tehran investigators Read more

The UK prime minister, Boris Johnson, later called for a full investigation into the crash and the reports it could have been caused by something such as a stray missile, following a telephone conversation with the Ukrainian president, Volodymyr Zelenskiy, Downing Street said on Thursday.

In the phone call, Johnson offered his condolences, offered UK support and was updated on the crash, a No 10 spokesman said. “The prime minister said that there needed to be a full, credible and transparent investigation into what happened.”

Asked whether this meant the UK was concerned about the possible cause of the crash, he said: “I’m not going to speculate on this, but the reports we have seen are very concerning, and we’re urgently looking into them.”

Images have been widely circulated online by an Iranian activist showing the remains of what could be a Russian-made Tor-M1 missile that he claimed was found near the plane’s crash site – but there was no way to independently verify that the debris in the picture shows a missile or if it was taken in the debris field on Wednesday.

The Ukrainian president, Volodymyr Zelenskiy, said earlier on Thursday that his government was considering “several possibilities” but asked people to refrain from speculating about the crash. Iranian authorities have blamed mechanical issues, which they said happened shortly after the plane left Tehran’s main airport at 6.13am on Wednesday.

The timing of the disaster, a few hours after Iran fired ballistic missiles at US forces stationed in Iraq in retaliation for its assassination of general Qassem Suleimani, has fuelled speculation that it might have somehow been linked to the hostilities, but so far there has been no evidence to back this up.

Aviation and security experts have said it is unlikely the plane could have been shot down near Tehran without a missile, or the rocket battery from which it was fired, leaving traces.

Justin Bronk, a research fellow at the Royal United Service Institute, previously told the Guardian that he believed on balance the incident was “a tragic coincidence”.

He said it would require a fairly large surface-to-air missile to inflict such catastrophic damage on a civilian airliner and “there was no evidence of a rocket plume” in the videos that have emerged so far. “It would also be very hard to conceal such a large rocket battery from the ground,” he added.

Iranian authorities have also rebuffed speculation the aircraft was attacked as part of a hostile misinformation campaign perpetrated by the country’s enemies.

A preliminary report issued by Iranian investigators into the crash late on Wednesday said the aircraft never made a radio call for help and was trying to turn back to the airport when it went down.

Facebook Twitter Pinterest Wreckage at the site of a crash, about 30 miles south of Tehran, Iran. Photograph: UPI/Barcroft Media

Witnesses to the crash on the ground and from passing aircraft said they saw the jet burst into flames while still in the air, according to the report, which stated technical problems were the most likely reason.

It confirmed that both the aircraft’s black boxes had been recovered, giving investigators access to data and cockpit communications, though some parts of their memory had been damaged in the crash.

Iran’s aviation authority said on Wednesday it would not hand over flight recorders either to the aircraft’s American manufacturer or to US aviation authorities, but that it would give Ukrainian investigators access to the investigation.

But the head of the authority, Ali Abedzadeh, appeared to backtrack on Thursday, saying claims in the Iranian media that the black boxes would not be sent overseas were “a mistake taken by the reporter”.

Zelenskiy said Ukrainian officials arrived in Tehran early on Thursday and that he would speak to the Iranian president, Hassan Rouhani, to ensure close cooperation.

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians, 11 Ukrainians and three Britons.

The Canadian prime minister, Justin Trudeau, offered his condolences to the families of the victims on Wednesday. “Our government will continue to work closely with its international partners to ensure that … [the crash] is thoroughly investigated, and that Canadians’ questions are answered,” he said.

The 737-800 belongs to the same family as, but is a different model to, the 737 Max 8 aircraft, which has been grounded since two fatal crashes occurred within six months in Indonesia and Ethiopia in 2018.

The 737-800 operates with a different software system to the one implicated in the Max 8 crashes.

The plane had been last serviced two days before the accident, according to the airline.

Zelenskiy on Thursday invited Britain to join the investigation in a telephone call with the prime minister, Boris Johnson. “The President of Ukraine invited the United Kingdom to join the investigation. Boris Johnson supported this idea and stressed that the best British experts should be involved in finding out all the circumstances of the tragedy,” his office said in a statement.

Under international aviation rules, Iran is responsible for running the inquiry.