Ukraine's President has vowed to "find out the truth" behind the crash of a Ukrainian jetliner that killed all 176 people on board shortly after taking off from the Iranian capital Tehran.

Key points: Iran says it will not return the plane's black box to Boeing

Iran says it will not return the plane's black box to Boeing Ukraine has declared January 9 a national day of mourning

Ukraine has declared January 9 a national day of mourning Many of those killed are believed to have been international students en route to Canada

President Volodymyr Zelenskiy said the Government was considering several possible causes of the crash, which killed 82 Iranians, 63 Canadians, 11 Ukrainians, 10 Swedes, three Germans and three Britons, among others.

The crash came hours after Iran launched a ballistic missile attack on Iraqi bases housing US soldiers, but Iranian officials said they suspected a mechanical issue brought down the three-year-old Boeing 737-800 aircraft, which was bound for Kiev, Ukraine.

"Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash," Mr Zelenskiy said.

Among those killed in the crash were 11 Ukrainians. ( AP: Efrem Lukatsky )

"We will surely find out the truth. For this purpose, a thorough and independent investigation will be conducted in accordance with international law."

While the cause of the tragedy remains unknown, the disaster could further damage Boeing's reputation, which has been battered by the furore over two deadly crashes involving a different model of the Boeing jet, the much-newer 737 Max, which has been grounded for nearly 10 months.

Mr Zelenskiy laid flowers at the airport of Boryspil, where the crashed plane was based, and declared January 9 a day of national mourning.

In a television statement, Mr Zelenskiy asked people to refrain from manipulation, speculation, conspiracy theories and hasty evaluations regarding the crash.

Mr Zelenskiy said he would be calling his Iranian counterpart. ( AP: Ukrainian Presidential Press Office )

Ukrainian crash investigators have arrived in Iran to assist in the probe, he said.

He also said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.

The head of Iran's civil aviation authority, Ali Abedzadeh, earlier told local media that the country would not be returning the black box to Boeing or US authorities.

"We will not give the black box to the manufacturer and the Americans," he was quoted as saying.

"This accident will be investigated by Iran's aviation organisation but the Ukrainians can also be present."

Iranian investigators claimed that crew members of the plane never made a radio call for help and were trying to turn back to the airport when the plane went down.

Members of Toronto's Iranian community hold a vigil for victims of the plane crash. ( AP: Chris Young via The Canadian Press )

Investigators also said that the plane was engulfed in fire before it crashed.

They said the crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel.

Canadian Prime Minister Justin Trudeau, meanwhile, vowed his government would get answers.

Mr Trudeau said 138 of the passengers were bound for Canada. The manifest listed several teenagers and children, some as young as one or two.

"Our government will continue to work closely with its international partners to ensure that this crash is thoroughly investigated, and that Canadians' questions are answered," Mr Trudeau said in a statement.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s "Canadians deserve answers": Canadian PM calls for crash investigation

"There is a clear need for answers … Canada is very concerned on this."

Canada broke off diplomatic relations with Iran in 2012, but Mr Trudeau said he expected Canada to play a big role in Iran's probe into the plane crash.

Mr Trudeau said Canadian Foreign Minister Francois-Philippe Champagne would call his Iranian counterpart Mohammad Javad Zarif to underline the need for a proper probe of the crash.

Many of the passengers were believed to be international students attending universities in Canada making their way back to Toronto via Kiev after visiting family during the winter break.

ABC/wires