There are reports that the Pentagon has asked the Conservative government to consider leaving a contingent behind in the war-ravaged nation to help train Afghan commandos and to keep up the fight against al-Qaida and Taliban militants.
Harper said there has been no specific request that Canada stay on past 2014, but suggested he may be willing to consider the idea.
"As we approach that date, we will examine all options and we will take the decision that is in the best interest of this country and in the best interest of our security objectives for the globe," Harper said under questioning in the House of Commons on Wednesday.
His refusal to rule out a further extension outraged New Democrats, who described it as a violation of the parliamentary motion that ended the country's combat mission in Kandahar a year ago.
Special forces have been operating in Afghanistan, alongside U.S. troops, since 2001. As with the regular army, their combat mission ended last summer.
A small contingent of the elite soldiers are among the 950 trainers Canada has provided to the continuing NATO training mission in Kabul and are also scheduled for withdrawal in the spring of 2014.
Opposition Leader Tom Mulcair says the public has been clear, they want the troops home and the NDP will fight any proposed extension.
Parliament has spoken and the country's role is finished, Mulcair said.
"Stephen Harper is playing with fire, once again. We should be gone from Afghanistan. It is over.
"It's not proper for a Canadian prime minister to be so subservient to American military dictates as Stephen Harper seems to be in this case, once again."
The prime minister said the NDP's reaction was "knee-jerk" and "ideological."
"It's not a remarkable statement that the NDP won't support the mission," said Harper. "The NDP couldn't even make up its mind to support World War Two."
There is concern among NATO allies that Afghan forces will not be prepared to handle security threats by the time the bulk of foreign troops leave.
Washington has reportedly struck a draft strategic partnership with the government of President Hamid Karzai, which would tie it and possibly other allies into a security and economic assistance support role for a decade beyond 2014.
The International Security Assistance Force is preparing to hand over the major share of combat operations to the Afghans within the next year, allowing them to plan and carry out raids against militant strongholds.
But relations have been strained almost to breaking point by a series of incidents, including the massacre of 16 villagers in Kandahar province by a U.S. soldier and the burning of a number of Qur'ans at an American base near Kabul.
There have also been a growing number of NATO troops shot by Afghans they were training.
It is no secret that Karzai wants foreign troops out. In response to a series of deadly attacks in Kabul, he recently chastised his "brothers" the Taliban by saying their actions would prompt western soldiers to stay longer.
Interim Liberal leader Bob Rae, who was instrumental in getting his party to accept Canada's participation in the current training mission, said the country has already done more than its fair share with 158 deaths, over 600 wounded and over 1,000 other non-battle casualties.
He said it's time to "take off the training wheels" with the Afghan government.
"I think anybody looking at the situation in Afghanistan would say clearly, the government of Afghanistan simply has to step up to take on its responsibilities," Rae said.
"I think we've reached the point now where it's clearly in Canada's interests to look to what can we do in terms of foreign assistance, what can we do in terms of diplomacy, what can do in terms of beefing up the capacity of our missions in Islamabad and Kabul so they can do a job of working with the government of Afghanistan.
"But I think we made decisions with respect to 2014. I think that's where we should be."
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