Ukraine's government vowed Friday to bring to justice those who downed a Ukrainian passenger jet in Iran, as hundreds gathered in Kiev to mourn the victims on the first anniversary of their deaths.

On January 8 last year Ukraine International Airlines (UIA) Flight PS752 crashed shortly after taking off from Iran's capital Tehran  killing all 176 people aboard, including 55 Canadians.

The Islamic republic admitted three days later that its forces mistakenly shot down the Kiev-bound Boeing 737-800 plane after firing two missiles, amid heightened US-Iran tensions.

On Friday, relatives, friends and colleagues of the Ukrainian crew and passengers attended a ceremony at the site of a future memorial to the victims on the banks of the Dnipro river.

"I assure you that the prosecutor's office and Ukraine's law enforcement agencies are doing and continue to do everything to establish the truth and bring the perpetrators to justice," deputy prosecutor general Gyunduz Mamedov told the victim's relatives during the ceremony.

Portraits of the victims were displayed on a large screen and tearful mourners laid flowers at a stone bearing a plaque inscribed with "PS 752" and the date of the downing.

"I wish none of this had happened," German Kozionov, a UIA pilot and friend of the ill-fated flight's crew members, told AFP. 

"I still think of them as living people. Good people who lived."

On Friday Ukrainian President Volodymyr Zelensky said on Facebook that "we are moving towards" bring those responsible to justice and securing compensation payments.

"The shelf life of such crimes cannot expire," he wrote.

Human Rights Watch said Friday that Iranian authorities have failed to conduct a transparent and credible investigation into the shooting down of Flight 752.

It added that more than a dozen of the victims' families had told the watchdog that Iranian authorities had "intimidated and harassed" them to stop them from seeking justice outside of official probes.

Canadian Prime Minister Justin Trudeau last month called on Tehran to answer questions about the downed plane after an independent Canadian report complained that Iran was "investigating itself, largely in secret".

Ukraine officials confirmed this week they received on December 31 a preliminary "technical report" from Iran on the circumstances of the disaster.

They have two months to review the document and decide if they are satisfied. 

Tehran has also offered to give $150,000 to the families of each of the victims.

But UIA chief Yevgeny Dykhne this week joined widespread criticism of the offer, dismissing it as a "media strategy just designed to test our reaction".