Lawmakers in Crimea voted unanimously to split from Ukraine and join Russia, and will hold a referendum March 16 to allow voters on the disputed peninsula to weigh in on the decision.
Harper described Russia's invasion of Ukraine as an act of aggression and a clear violation both of Ukraine's sovereignty and international law.
The prime minister says Canada continues to view the situation in Ukraine "with the gravest concern" and will co-operate with its G7 partners and like-minded allies.
Moscow has so far refused to withdraw its troops from the strategic region, which also houses Russia's Black Sea fleet — a tense standoff that has triggered international sanctions against Russia and visa restrictions on its officials.
Ukraine's prime minister has called the Crimean lawmakers' decision illegitimate, but Russia says if Crimea votes to become part of Russia, they would introduce legislation to speed up the procedure.
Foreign Affairs Minister John Baird said it's impossible to organize a meaningful referendum in the 10-day time frame proposed.
He said Canada is deeply concerned about Russia's military actions.
"This is a vestige of another century," he said. "It's a Soviet-style tactic that's unacceptable for a G8 country and unacceptable in 2014.
"We're going to condemn it in the strongest of terms and work with friends and like-minded allies to see it reversed.
"It's going to be a challenging issue."
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