His remarks, in Quebec City, came as the Israeli military prepared for a possible ground incursion into the Palestinian-controlled Gaza region, which observers say could lead to a dramatic escalation of the conflict.
Aside from the call-up of reservists, the Israeli Defence Force announced Friday that a highway leading to Gaza would be off-limits to civilian traffic, according to published reports in the region.
Rockets fired by Hamas militants were aimed at Tel Aviv and Jerusalem — two targets previously thought out of range of missiles from Gaza.
There is speculation in the defence community that militants are using Iranian-made Fajr-5 missiles.
Harper's government has been a staunch, outspoken backer of Israel, and recently won applause from Prime Minister Benjamin Netanyahu's government for the abrupt closure of the Canadian embassy in Tehran.
"We recognize and support Israel's right to defend itself against such terrorist attacks, but obviously we urge all sides to take all precautions possible to spare any innocent lives," Harper said Friday.
Three Israelis were killed in the bombardment Thursday, while published reports in the Middle East claim the death toll among Palestinians has hit 22.
The uncertain climate was underscored at an international defence symposium in Halifax, where Defence Minister Peter MacKay described volatility in the Middle East and elsewhere "as the new normal."
"The bottom line is that there is a complex interplay of events and trends; threats and challenges … confluences that can create unforeseen outcomes," MacKay told participants at the invitation-only event.
He also warned against Iran's nuclear aims, which the government in Tehran characterizes as peaceful but many other nations, including Israel, see as an attempt to acquire nuclear weapons.
"We continue to bear witness to atrocities across the Middle East and the possibility of even greater atrocities if Iran successfully builds a nuclear bomb," said MacKay.
The unfolding crisis drove oil prices up by one per cent on Friday.
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