OTTAWA, July 5 (Reuters) - The deaths of six Canadian soldiers in Afghanistan are particularly troubling but the country's military mission will stay in place until early 2009 as scheduled, Prime Minister Stephen Harper said on Thursday.

The six died when their vehicle hit a roadside bomb on Wednesday. Canada has now lost 66 soldiers since sending troops to Afghanistan in 2002, most of them in the last 18 months.

"Casualties at this level weigh very heavily upon my mind," Harper told a nationally televised news conference in Halifax, Nova Scotia. "Obviously, casualties weigh very heavily on the government."

Canada's 2,600-strong mission in the southern city of Kandahar had been due to leave in February 2007. Last year the Conservative government forced a motion through Parliament extending the departure date until February 2009.

"I think this government has been very clear about the duration of this mission. Parliament has approved that to February 2009," said Harper, who did not respond when asked whether he would press NATO for other members to take more of the load.

"Obviously, today is not the day to have a political debate on the future of the mission," he said.

Harper, whose Conservatives control only a minority of seats in the House of Commons, says there would have to be general political agreement for the mission to be extended beyond 2009. Opposition parties have already ruled this out.

Harper received support from an unlikely source when the centrist Toronto Star newspaper -- no fan of the Conservatives -- said the soldiers were doing a worthwhile job.

"Canada is on the right mission, costly as it is, to buy time for (Afghan) President Hamid Karzai's democratically elected government to gather its strength," the Star said in its lead editorial on Thursday.

The Star is Canada's largest circulation newspaper.

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