Prime Minister Stephen Harper said he is "deeply saddened" by the loss of four Canadian soldiers, killed in Afghanistan on Sunday during a fierce battle with insurgents.

"We are proud of these soldiers' contribution to bring stability and hope to the people of Afghanistan," said Harper in a statement.

"These soldiers lost their lives in the service of their country. Canada is grateful for that service, and saddened by this loss."

His statement came hours after NDP Leader Jack Layton repeated his call for ending the Afghanistan mission in February 2007.

"Young people have stepped forward to put their lives on the line, fulfilling a mission that they were asked to fulfill," Layton told reporters in Toronto.

"What we as Canadians need to do is consider whether this is indeed the right mission for Canada going forward. Our view is that it is the wrong mission."

Layton had earlier criticized the Conservative government for having no exit strategy in Afghanistan, and said the focus has changed from reconstruction to open war.

Troops are currently engaged in Operation Medusa -- a major offensive in the violent Panjwaii district, to combat insurgents in Taliban strongholds.

NDP Defence Critic Dawn Black argued that focusing on wiping out the Taliban is not helping the Afghanistan people.

"The extension of the mission for another two years in this kind of warfare is not achieving the kind of results that we want to see for the people of Afghanistan," Black told CTV Newsnet.

"It's no a balanced mission. There's nothing happening there in terms of development or diplomacy at this point. And we don't believe that you can make a positive change in the lives of the people in southern Afghanistan by simply bombing and killing (insurgents).

"We understand you need a defensive role to bring peace and increased security, but you must also be doing the development and diplomatic work that leads to an enduring security."

Liberal Defence Critic Ujjal Dosanjh attacked the NDP for what he called poor timing in their criticism, and said the mission should not be politicized.

"When you have a major offensive going on against the Taliban, it's not a time for politics," Dosanjh told CTV News. 

At least two of the soldiers killed on Sunday were based in CFB Petawawa in eastern Ontario: Warrant Officer Frank Robert Mellish and Warrant Officer Richard Francis Nolan.

There are 5,000 military families at the base, and CTV's David Akin said that "you will hear nary a word of dissent about the nature of the mission from them here ... they are committed to it, and they realized the dangers and the risks.

He added that there were yellow ribbons displayed throughout the base, to signify the families were thinking about the troops and hoped for their safety.

With reports from CTV's David Akin and Roger Smith