Questions are being raised about the Conservative government's procurement of Russian helicopters that Canadian pilots have been secretly using to fly troops into combat in Afghanistan.

Until this week, the government had been silent about the MI-17 "Hip" helicopters that were leased last year. The government still refuses to provide any details of their procurement, including how much the lease cost.

The federal government has been silent about the MI-17 "Hip" helicopters that were leased last year. (Bill Graveland/Canadian Press)

"It was competed, it was open, but for reasons of security I really can't go into any other details," Defence Minister Peter MacKay said Wednesday.

Military sources told CBC News that the idea of leasing Russian choppers was approved by cabinet early last year. It took some time to train Canadian crews, but the helicopters went into service quickly, used by Canadian special forces troops on secret missions.

Over time, their use expanded to include regular soldiers on regular missions, sources said. The military said the Russian choppers are "very robust" and "very capable."

"What's surprising about this is the fact they made a big secret about it," said NDP defence critic Jack Harris.

Harris said if the Russian choppers are good enough for Canadian troops at war, why did the government rule them out when it decided in 2006 to buy new U.S. helicopters for the military.

At the time, the government said, there were no other choppers capable of doing the job. The American Chinooks are larger, but the MI-17s appear to be just as capable in the air above Kandahar.

They also come with a built in de-icing system — a perk Canada had to pay extra for on its Chinooks. The cost of buying a single Chinook works out to about $80 million, compared with $17 million for one of the Russian helicopters.

"So, we've got an out-of-control department of National Defence when it comes to procurement," Harris said.

Defence analyst Rob Huebert said the huge price difference between the two helicopters might help explain why the government has kept the deal secret.

"From a political perspective, one can also see that the Conservatives may not want to be seen to be undermining their claim that they needed the Chinooks to the degree that they did," Huebert said.

But he said the air force was wise to choose an American helicopter to be a permanent part of the Canadian equipment in order to have access to experts or spare parts. That could be a concern if relations with Russia start to freeze up, he said.