Iran has invited the UN's International Civil Aviation Organization (ICAO) to provide expert advice to support an investigation into the downing of a Ukrainian International Airlines Boeing 737 near Tehran.

The Iranian government is facing tough questions over why its military took several days to admit to accidentally shooting down the plane, following repeated denials of involvement.

On Monday, protesters took to the streets of Iranian cities for the third day over the military's role in the crash on January 8 that killed 176 people. Iranian officials have denied any cover-up. 

In a statement Monday, the ICAO said it will be sending expert technical staff to assist in investigating the circumstances of the crash. The ICAO will be joined by several teams of experts, including from airline manufacturer Boeing and Ukraine. The Canada-based ICAO is not typically involved directly in investigations, but generally plays an advisory role.

Read more: Opinion: We need transparency from Tehran

One of the ICAO's primary tasks will be to ensure the investigation is carried out in compliance with international civil aviation standards. The organization says it serves as a forum for cooperation in all fields of civil aviation for 193 member states.

Canada wants answers

Fifty-seven of the victims held Canadian passports, and many were Iranian-Canadian students and academics heading home after holiday visits. Ottawa has already sent a team of air crash investigators to Tehran. 

On Monday, Canadian Prime Minister Justin Trudeau said the crash victims would still be alive and home with their families if there were no tensions in the region.

In an interview with Global television, Trudeau said the international community has been "very, very clear about needing to have a non-nuclear Iran" but also in "managing the tensions in the region that are brought about by US actions as well."

'Disastrous mistake'

Speaking with Iranian President Hassan Rouhani over the weekend, Trudeau called for "a full and complete investigation" and for Iran to accept "full responsibility."

On Saturday, Rouhani said the downing of the Ukrainian International Airlines jet was a "disastrous mistake."

Tehran said that air defenses were fired in error while on alert after Iranian missile strikes on US targets in Iraq, and that the airliner was mistaken for a "hostile target" after it turned toward a sensitive military base of the elite Revolutionary Guards.

Read more: Germany, world leaders react after Iran admits downing plane by 'mistake'

Representatives of five nations whose citizens died in the crash will meet in London on Thursday to discuss possible legal action, according to Reuters news agency.

wmr/cmk (dpa, Reuters)

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