Harper said Hamas is solely responsible for the death and destruction in Gaza following almost a month of fighting between Israel and Hamas militants.
His remarks, made following a job training announcement in northern Saskatchewan, came as the United States and the United Nations condemned the shelling of a UN school that was sheltering displaced Palestinians on the Gaza Strip. At least 17 people were killed.
"Obviously no one likes to see the suffering and loss of life that has occurred," Harper told reporters in Air Ronge.
"That said, we hold the terrorist organization Hamas responsible for this. They have initiated and continue this conflict and continue to seek the destruction of the state of Israel."
Some 3,300 Palestinians, including many women and children, were taking refuge in the school building in the Jabalya refugee camp when it came under fire around dawn on Wednesday, said the United Nations Relief and Works Agency.
The attack prompted the U.S. to condemn the shelling, though it did not single out Israel by name.
"We are extremely concerned that thousands of internally displaced Palestinians who have been called on by the Israeli military to evacuate their homes are not safe in UN designated shelters in Gaza," the White House added in a statement.
UN Secretary General Ban Ki-Moon called the attack "shameful" and said all available evidence points to Israeli artillery as the cause. The Israeli military said it fired back after its soldiers were targeted by mortar rounds launched from the vicinity of the school.
Harper, meantime, suggested Israel has been justified in its actions throughout the conflict, saying that if a terrorist organization were attacking Canadians, there would be a similar response from Canada.
Unprompted, Harper also had harsh words for Russia over the continuing conflict in Ukraine.
"There is zero doubt that those people who are responsible for violence and destabilization in Ukraine are acting as agents of the government of Russia," he said. "Russia's conduct in this is unacceptable."
He added that there's "no doubt ... the agents that Russia has armed" downed a Malaysian Airlines passenger jet two weeks ago.
Harper also took aim at China, saying once again that there was "no doubt" that the Chinese were behind the cyberattacks on Canada's federal scientific research agency.
Earlier this week, the federal government's chief information officer, Corinne Charette, said the National Research Council was the target of a cyberattack from a "highly sophisticated Chinese state-sponsored actor."
The Chinese embassy in Ottawa denied being involved in any such attack. But Harper is apparently not buying it.
"We have obviously experts in the cyber-security field that are monitoring this very closely," he said.
"It's an ongoing battle. They tell me there is no doubt as to the source of this particular attack, and this obviously is being taken up with the government of China at the highest level."
With files from The Associated Press
Also on HuffPost