Iran's civil aviation authority has said it's following international rules and will allow other countries to participate in its investigation of a plane crash that killed dozens of Canadians.

But the role Canada is being offered by Tehran amounts to the bare minimum required by the international legal convention on aviation accident investigations — and at this point does not include active participation in the probe into the crash of Ukraine International Airlines Flight PS752 on Wednesday.

Prime Minister Justin Trudeau said Thursday that Canada is working with Ukrainian investigators and continuing to ask Iran for more access for its experts to help conduct an in-depth and credible investigation.

"The families of the victims and all Canadians want answers," said Trudeau. "I want answers. That means closure, transparency, accountability and justice."

'I think it is too soon to be drawing conclusions," Trudeau says. 0:31

The Convention on International Civil Aviation, signed by 193 governments around the world, includes standards and requirements for international co-operation on aircraft accident investigations.

Since Canadians died onboard Flight PS752 in Tehran, Canada has some rights that kick in automatically, according to the international requirements.

The Transportation Safety Board (TSB) is allowed to appoint an expert to the investigation. The agency already has designated an expert who can visit the scene of the crash.

The TSB confirmed Thursday that Iranian authorities have invited it to the crash site, and the TSB is "making arrangements" to travel there and work with "other groups and organizations already on site."

Canada's designated expert also will be able to receive and review information released by Iran's aviation officials, monitor the progress of the investigation, and receive a copy of the final report according to the convention.

'These families need their answers'

David McNair is a former veteran Canadian aviation safety investigator with the TSB. He said the level of access to the investigation Iran is offering is "quite limiting for countries who lost citizens."

"They don't get information on the witness interviews, the flight data recorder information," said McNair. "They're quite limited ... A lot of questions have to be answered and you have to wait for [the] investigating authority to release information.

"These families need their answers and they need them quickly. It's a very tough situation."

The TSB can ask for additional information and offer the Iranians its expertise. But there's no guarantee that offer will be taken up, said the TSB.

Foreign Affairs Minister François-Philippe Champagne has put pressure on his Iranian counterpart to grant Canada deeper access and permit it to take an active role in the investigation. Canada cut diplomatic ties with Iran in 2012. The Trudeau government has since tried without success to re-establish diplomatic relations.

Ukraine accepts Canadian offer for aviation expertise

The State Aviation Administration of Ukraine is also investigating the accident, since the aircraft belonged to a Ukrainian carrier.

Transport Minister Marc Garneau's office confirmed Ukraine has accepted Canada's offer for civil aviation expertise. What's still not clear is whether Iran has approved Canada as part of Ukraine's larger investigative team, according to his office.

Ukrainians have been advocating for this move on the ground. Transport Canada said it has expertise in aircraft design, maintenance and flight operations, and is ready to assist.

Trudeau said Iranian authorities told Ukraine's president that its investigators would have access to the plane's black boxes that are staying in Iran.

WATCH: Ukraine mourns, sends investigators to Iran

A day of mourning was declared in Ukraine as the country sends crash investigators to Iran to determine the cause of the crash. 1:56

If Iran accepts Canada's request for clearance to help with its lead investigation, it would give Canada access to a "second tier" of the investigation. According to the international rules, an invitation could entitle Canadian authorities to participate in all aspects of the investigation under Iran's investigator-in-charge. That access would include:

Visiting the scene of the accident.

Examining the wreckage.

Obtaining witness information and suggesting areas of questioning.

Full access to all relevant evidence as soon as possible.

Copies of all pertinent documents.

Participation in off-scene investigative activities, including examinations, tests, simulations and technical briefings.

Participation in investigative progress meetings, including deliberations related to analysis, findings, causes, contributing factors and safety recommendations.

Canada also could make its own submissions to the investigation. The final investigation report is supposed to be completed within a year.

WATCH: The challenge of investigating a plane crash with Iran