- Video report by ITV News Correspondent Sejal Karia
Three Britons have been named among those killed in a plane crash moments after it took off from Iran's biggest airport.
Sam Zokaei from Surrey, Saeed Tahmasebi Khademasadi from west London and Mohammad Reza Kadkhoda Zadeh from near Brighton have been confirmed among those who lost their lives.
All 176 people on board died when Ukraine International flight PS752 crashed in the early hours of Tuesday at Imam Khomeini International Airport in Tehran.
Local officials have blamed an engine fire, whilst an international investigation is working to establish the cause of the disaster - an effort mired by US sanctions against the Middle East theocracy.
Ukraine's foreign minister, Vadym Prystaiko, said there were 82 Iranians, 63 Canadians and 11 Ukrainians on board - including nine crew members - as well as 10 Swedish, four Afghan, three German and three British nationals.
University pays respect to 'brilliant' engineer killed in crash
Tributes have been paid to the three British nationals killed in the crash.
A spokesman for Imperial College London, where Mr Tahmasebi was a post-graduate researcher, said: "We are deeply saddened at this tragic news.
"Saeed Tahmasebi Khademasadi was a brilliant engineer with a bright future.
"His contributions to systems engineering earned respect from everyone who dealt with him and will benefit society for years to come.
"He was a warm, humble and generous colleague and close friend to many in our community."
Staff at BP, Mr Zokaei's employer, also paid tribute to their 42-year-old colleague.
A spokesman said: "We are shocked and deeply saddened by this tragic loss of our friend and colleague and all of our thoughts are with his family and friends."
There was no answer from the home of Mr Kadkhoda Zadeh's father in Haywards Heath, near Brighton.
The 40-year-old is listed as owning a dry cleaning business.
Staff at nearby Hassocks Pet Centre remembered "a lovely man [with] a lovely smile".
Investigation into crash now taking place
Air crash investigators are now attempting to establish what happened to the Boeing 737-800 aircraft.
Open source data shows the flight took off after 6am local time on Wednesday morning, but stopped sending communications almost immediately after.
Journalists who were at the site of the crash reported bodies being strewn across farmland among the smoldering debris of the aircraft.
State media has reported that Iran will not give the black box from the crashed plane to manufacturer Boeing, an American company.
The flight recorders, known as black boxes despite their orange colour, provide key information for investigators about the final moments of aircraft before serious crashes.
The flight data recorder (FDR) will provide a record of how the plane's systems operated and the actions of the pilots over several hours prior to the crash.
From the cockpit voice recorder, investigators may also be able to determine more about the last actions of the pilots, who did not send a mayday warning.
Under International Civil Aviation Organisation rules, investigations into air crashes are led by the country where the accident took place but other countries may be involved including the country of the manufacturer.
On Wednesday, the head of the Iran Civil Aviation Organisation, Ali Abedzadeh, was quoted as saying that Iran would not give the flight recorder to US-based Boeing or US authorities.
Mr Abedzadeh added that it is unclear which country the black box will be sent to for investigation.
Ukraine is expected to be involved in the process.
Iranian state media has reported both recorders have already been recovered from the wreckage.
The crash came hours after Iran launched a ballistic missile attack targeting two bases in Iraq housing US forces in retaliation for the killing of Revolutionary Guard General Qassem Soleimani, although Iranian officials were quick to dissolve any link between the two.
Qassem Biniaz, a spokesperson for Iran’s Road and Transportation Ministry, said it appears a fire had started in one of the plane’s engines.
The pilot then lost control of the plane, sending it crashing into the ground, Mr Biniaz said, according to the state-run IRNA news agency.
Ukraine opens criminal proceedings into crash as airline suspends flights
Ukraine’s president Volodymyr Zelenskyy said he had ordered the Prosecutor General to open criminal proceedings.
Speaking on Wednesday morning, the former TV comedian said: "An investigation commission should be set up of representatives of the civil and aviation agencies responsible for civil aviation.
"We have to work out all the possible versions.
"Regardless of the conclusions regarding the causes of the Iranian catastrophe, the airworthiness of the entire civilian fleet will be tested.
"I keep all measures on personal control.
"I very much ask everyone to refrain from speculation and putting forward untested versions of the disaster before the official announcements."
Meanwhile, Ukraine International Airlines said it had suspended flights to Tehran until further notice.
It expressed its "deepest condolences" to those who had lost their lives and their families.
What do we know about the aircraft that crashed?
The Boeing 737-800 is a very common single-aisle, twin-engine jetliner used for short to medium-range flights.
Thousands of the planes are used by airlines around the world.
Introduced in the late 1990s, it is an older model than the Boeing 737 MAX, which has been grounded for nearly 10 months following two deadly crashes.
A number of 737-800 aircraft have been involved in deadly accidents over the years.
In March 2016, a FlyDubai 737-800 from Dubai crashed while trying to land at Rostov-on-Don airport in Russia, killing 62 on board.
Another 737-800 flight from Dubai, operated by Air India Express, crashed in May 2010 while trying to land in Mangalore, India, killing more than 150.
Chicago-based Boeing was "aware of the media reports out of Iran and we are gathering more information," spokesperson Michael Friedman told the Associated Press.
Boeing, like other airline manufacturers, typically assists in crash investigations.
However, that effort in this case could be affected by the US sanctions campaign in place on Iran since President Donald Trump unilaterally withdrew from Tehran’s nuclear deal with world powers in May 2018.
Both Airbus and Boeing had been in line to sell billions of dollars of aircraft to Iran over the deal, which saw Tehran limit its enrichment of uranium in exchange for the lifting of economic sanctions.
But Mr Trump’s decision halted the sales.
Under decades of international sanctions, Iran’s commercial passenger aircraft fleet has aged, with air accidents occurring regularly for domestic carriers in recent years, resulting in hundreds of casualties.