Ottawa's apparent refusal to grant visas to the most senior Russian and Chinese delegates at a prestigious international astronautical conference Monday blindsided the head of Canada's space agency, who was left struggling to come up with an explanation.

The leaders of Russia's and China's space agencies were conspicuous by their absence at the opening plenary session in Toronto, sparking questions from among the thousands of participants.

The questions initially landed in the lap of Berndt Feuerbacher, past president of the International Astronautical Federation, who was moderating the session featuring the heads of space agencies.

"They were foreseen to be here with us, they have been with us in the past, and they will be with us in the future," Feuerbacher said.

"It is just unfortunate, due to problems especially in the visa area, that we couldn't have these delegations here. I apologize for this."

The issue came amid much delegate talk about the importance of international co-operation in the space-exploration field as symbolized by the International Space Station. Russia plays a key role in the space station — its capsule brought home Canadian astronaut Chris Hadfield — but has drawn the wrath of countries such as Canada and the U.S. for its aggression against Ukraine.

"How do you want to succeed talking about global space co-operation without involving representatives from Russia and China?" delegates asked Feuerbacher.

"This is not our intention," he responded.

Walter Natynczyk, head of the Canadian Space Agency, was unable to explain what had gone wrong.

The retired general said he had only been alerted to the visa problem in the past couple of days.

"I guess I'm equally surprised," Natynczyk told The Canadian Press.

"Before I come to any conclusions, I would like to know exactly how it transpired."

Delegates from about 70 countries expected to attend

A spokeswomen with Citizenship and Immigration Canada cited privacy laws in refusing to discuss "whether delegates from certain countries have or have not been refused visas" for the conference.

The Canadian Aeronautics and Space Institute is hosting the showcase 65th International Astronautical Congress. About 3,000 delegates from space agencies, companies, academia and government from about 70 countries around the world, including France, South Africa and Mexico, have descended on the downtown convention centre.

The program features a welcome message from Prime Minister Stephen Harper, who touts Canada's "enviable history" of putting space to use for Canadians and humankind.

It also features photographs of Russia's Oleg Ostapenko and China's Xu Dazhe as participants at the head of agencies session.

Geoff Languedoc, the institute's executive-director, said last month that invitations had gone out to the head of the Russian and Chinese space agencies and he expected they would be there.

While the Canadian Space Agency is apolitical, "international stuff is pretty complicated," Natynczyk said ruefully.

"Unfortunate that some folks aren't here, but I've no doubt that the collaboration and the partnership will carry on," he said.

"Notwithstanding the international events, communities of shared interests get together."

NASA Administrator Charles Bolden said space co-operation would go on regardless of political problems on Earth.

"Can we work given the political strains? Yes," Bolden said. "I would ask you to look at the International Space Station."