Nato Secretary General Jaap de Hoop Scheffer has urged Canada to keep its military mission in Afghanistan beyond the planned 2009 pullout date.
Canada's casualties in Afghanistan are mounting
Canada was playing a vital role in helping Afghanistan to rebuild, he said during a visit to Montreal.
Mr de Hoop Scheffer's request comes amid rising public disquiet in Canada about the country's Afghan presence.
Since 2002, 61 Canadians have died in Afghanistan, including three killed in a roadside blast this week.
"I know how dramatic it is if Canadian soldiers pay the highest price," Mr de Hoop Scheffer said.
"But I still say, you are there for a good cause... You are there to defend basic human values."
Canada has 2,500 troops deployed in southern Afghanistan as part of the 40,000-strong International Security Assistance Force.
This week, BBC News is taking an in-depth look at the challenges facing Afghanistan's people and the peacekeepers.
Stories include: the state of the Taleban; corruption; the drugs problem; and attacks on schools.
Canadian Prime Minister Stephen Harper, who heads a minority Conservative government, has previously indicated that the country's forces may have to stay beyond their current mandate.
But unless parliament votes to extend the mission, the Canadian troops are expected to return in 2009.
A rising number of Canadian casualties and allegations that Taleban prisoners were tortured after being handed over to the Afghan authorities has increased public unease over the mission, correspondents say.
There is also concern in Canada that its forces, together with those from the US, Britain and the Netherlands are bearing the brunt of fighting the Taleban in the most violent areas.
Other contributing countries, including Germany, France and Italy, have restricted their forces to the more peaceful northern areas.
Mr de Hoop Scheffer's visit to Montreal coincided with the publication of an opinion poll indicating that 70% of people in Quebec oppose the Afghan mission.
The Nato secretary general said he believed more time was needed to create conditions in Afghanistan for reconstruction and development to continue.
He said he hoped all members of the Nato alliance remained united in their resolve to quell unrest and rebuild Afghanistan.