It seems Canadian PM Stephen Harper may be forced to reconsider his word on maintaining the number of the country's military ranks.

On Tuesday, Gen Tom Lawson, Canada's chief of defence staff, said the posts of some 68,000 full-time and 27,000 part-time members of Canadian military may tether in December once Mr Harper gets to review an updated Canada First Defence Strategy.

Canadian Prime Minister Stephen Harper gestures as he speaks to the Montreal Board of Trade in Montreal, November 15, 2013. REUTERS/Christinne Muschi

The updated version of the Canada First Defence Strategy policy will attempt to strike a balance between shrinking budgets and the needs of the military, Gen Lawson said.

Canada's top soldier is being forced to do slashes on the number of military personnel owing to an appropriations budget that could shrink by up to $2.5 billion by 2014. Gen Lawson had been doing all the necessary balancing that he can - keeping the numbers while trying to maintain the ability, integrity and responsiveness of Canada's military.

"It is always an option, but the direction has not been given to us yet," Mr Lawson said on Tuesday after a speech at the Canadian Club in Ottawa.

"You have to provide all kinds of optionality to the government when affordability is an issue."

Part of the option is either to reduce the 68,000 full-time and 27,000 part-time members of military. Or give up some of Canada's artillery which include ships, planes or tanks.

"Certainly the government has not indicated a desire to cut numbers. We will see in this review if that is one of the things they un-pin," he said.

The budget cuts the central government want could not be achieved if the military continues to maintain its present numbers, according to retired general Rick Hillier, a former chief of defence staff.

"If we do this right, we can still have an agile force, we can still have a superbly trained force and we can still have a force capable in this era of threats," Mr Hillier told CTV in an earlier interview.

"But it's going to be smaller, you just can't get around it."

Andrew Leslie, another retired lieutenant-general Andrew Leslie, said Canada's National Defence can still achieve savings in ways other than slashing the number of military personnel.

"This year alone, (National Defence) has . . . spent over $3.1 billion on professional services, consultants and contractors," Mr Leslie said. "When we're talking about cutting people, let's start there."