OTTAWA - An additional 15,800 troops are needed to properly secure Kandahar so that reconstruction efforts and training of the Afghan army can continue, says a conservative U.S. think-tank.

A presentation by the American Enterprise Institute, widely circulated among western allies, painted a detailed, bleak picture of the American or NATO resources military experts believe are needed to win the fight against the Taliban.

The reinforcements would be in addition to the roughly 6,000 Canadian, American and Afghan combat troops already operating in Kandahar, one of Afghanistan's most volatile provinces.

The assessment comes as military planners in Ottawa examine options for the 2011 withdrawal of Canadian troops.

The Obama administration is close to deciding how many more troops to send to Afghanistan and the study, by Frederick and Kimberly Kagan of the institute's warfare centre, suggests the bulk of those reinforcement should go to Kandahar.

Moving around existing troops isn't an option.

"There are no forces to be redeployed in (Regional Command South) -- all are fully committed in tasks that cannot be abandoned," said the study, which has been examined at National Defence headquarters in Ottawa.

It reflects the private concerns of U.S. commanders on the ground who throughout the summer have expressed alarm at the strength of the insurgency in Kandahar. There has been some second-guessing of the decision to send the bulk of the first wave of American reinforcements to neighbouring Helmand province.

The enterprise institute, founded in 1943, is currently populated with a number of former Bush Administration officials.

Depending on how many soldiers Washington authorizes and the strategy NATO commander U.S. Gen. Stanley McChrystal employs, the study said Kandahar needs a bare minimum of one brigade of 3,500 additional troops. But the optimal number to carry out the strategy of securing the population would be 15,800 -- or up to five more combat brigades.

The report notes the planned withdrawal of Dutch and Canadian troops within the next two years would have to be factored in as well.

"Either the U.S. or NATO will thus have to find an additional brigade to offset those departures within the next two years," said the report, penned in mid-September.

Canada's long-articulated exit strategy in Kandahar has been to train the Afghan army to handle the country's security.

But the report found that because there is no rotation among Afghan units, the newly trained soldiers "are being ground up disproportionately."

And without reinforcements, "it is not at all clear that they will survive even the current level of strain."

The study quotes the commander of the 205th Afghan Corps based in Kandahar as asking: "Are you trying to build an enduring 1/8Afghan 3/8 Army or an expendable one?"

The suggestion that well over 20,000 soldiers are needed to secure the province where just over 2,000 Canadian troops have been fighting and dying for almost four years is a dramatic illustration of how badly Afghanistan has been let down by its NATO allies, says a retired Canadian general.

"The public should be upset and proud all at the same time," said former major-general Lewis MacKenzie.

"They should be proud that the battle groups have been able to hold their own and not lose Kandahar city because that's naturally high on the list of Taliban priorities."

But he said Canadians should seriously question and debate the country's future role in NATO once the Afghan mission is over.

"I think Canada should call for a post-Afghanistan conference, not necessarily to discuss what went wrong in Afghanistan because that'll be blatantly obvious, but on the future of the alliance."

Prime Minister Stephen Harper's government has repeatedly pleaded with NATO for reinforcements in Kandahar since late 2006. There have been promises, but very little relief.

Canada has lost 132 soldiers -- a staggering number considering the size of the force. If Canada's allies had faced the same casualty rate, it would have meant over 3,000 American and 600 British soldiers killed in action.

Had that been the case, MacKenzie said the public outcry in both the U.S. and the U.K. would have been furious. Rising casualty rates have already led to an erosion of public support in those countries.

MacKenzie said Canada needs NATO, but has cause to question if "it's going to work in the future" and whether allies would help if this country found itself in trouble.

"I'm not sure they'd be able to find their way across the Atlantic," he said.