The peace deal was hammered out following all-night negotiations between Russia, Ukraine, France and Germany at a summit in Belarus.
Part of the agreement includes a ceasefire that will be effective beginning this weekend.
"Obviously, we will take with some encouragement the fact that there is a ceasefire and some kind of an agreement in Minsk today," Harper said during a news conference in Victoriaville, Que.
"But we will remain skeptical of it."
Russian President Vladimir Putin will have to demonstrate he can abide by the new agreement, Harper said.
"To this point, Mr. Putin has systematically violated every agreement he has made to end violence, to end military action and to end aggression against Ukraine and Ukrainians," he said.
"We will judge him not by his words, not by his agreements — we will judge him by his actions."
The United Nations estimates 5,300 people have been killed since last April when Russian separatists began clashing with Ukraine's military.
Harper said earlier this week that Canada was hopeful for a diplomatic resolution to the Ukraine crisis, but reaffirmed Thursday that the government is keeping options open to help Ukraine should the situation escalate again.
Canada's current assistance to Ukraine includes non-lethal military aid.
"We will continue to explore options going forward for how we can help the Ukrainian people if they continue to face the kind of military aggression they have faced from Mr. Putin's regime," Harper said.
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