Ottawa must carry out its proposed multibillion dollar purchase of American-made F-35 fighter jets or it could risk causing problems in Canadian air force down the road, the defence minister says.

Peter MacKay, speaking at a press conference in Ottawa alongside U.S. Secretary of Defense Robert Gates, reiterated that buying 65 of the fifth-generation jets will help the Canadian military deal with "unforeseen threats" in future.

But scrapping the $16 billion purchase could cause an "operation gap" when the military's current fleet of fighter jets are retired later this decade, if an adequate replacement can't be found in time.

The development of the F-35 has drawn controversy both in Canada and in the United States. The Pentagon, which is the largest purchaser of the new jets, has aired concerns over rising costs and other production problems.

In Ottawa, Gates defend the program, describing Washington's "strong commitment" to the F-35 development program.

Gates did acknowledge there have been "some recent adjustments to the program," but said he was confident "that the F-35 will be the backbone of our tactical air force for years to come -- and I'm pleased that the Canadian military will make it theirs as well."

Ten countries are participating in the F-35 development program, under which more than 3,000 of the jets are expected to be built. Gates said the U.S. expects to buy between 1,300 and 1,400 of those.

Canada's participation also means domestic firms will have a chance to bid on contracts to help build the entire fleet, not just the 65 aircraft under consideration by Ottawa.

When he was asked about criticism of the F-35 by opposition parties, Gates said he did not want to comment on Canada's domestic affairs.

"I would just say that my hope is that for all of our sakes that all of our partners continue to move forward with us on this program," Gates said.

He also lauded the federal government's move to buy the F-35 because it would help cooperation at NORAD, the joint military entity responsible for defending North American air space.

After the meeting, Liberal Leader Michael Ignatieff renewed his party's concerns about the sole-source procurement for the planes, and the high cost during a time of fiscal austerity.

Gates "knows full well the problems with the F-35. This thing is late, it's over-budget," Ignatieff told reporters at an event in Montreal.

"What we're saying is we need to get a competitive bid here," he added. "We need to ask questions about whether this is the plane we need."

The F-35, which is being built by American defense firm Lockheed Martin, is known as a "fifth generation" fighter jet partly because it features stealth capabilities. The U.S. has the only stealth fighter currently in service, the F-22 Raptor.

On CTV's Power Play, Liberal Deputy Leader Ralph Goodale questioned whether Canada will need such advanced capabilities because, he said, the government's long-term military interests haven't been clearly articulated.

"Part of the problem is the government, has never -- and they didn't today -- they've never specified exactly the foreign affairs and defence mission for which this particular aircraft is uniquely suited," Goodale said.

"They don't define the mission and they won't expose it to competition so that we can get best value for money for Canadian taxpayers and for the Canadian military."

Afghan war

MacKay and Gates said the discussed a range of issues including the Arctic, narcotics and human trafficking.

Gates praised Ottawa's work in Afghanistan, noting that Canada has suffered more casualties per capita there than any other NATO country. He also described training Afghan security forces as "the pillar of our strategy and key to our ultimate success."

MacKay repeated that Canada's training mission will focus on the area around Kabul, far from the volatile south where Canadian troops have been fighting for years.

He said that 950 trainers will head to the Kabul area after Canada's combat mission ends this summer. They will work "behind the wire," or away from combat.

Military officials are hunting for locations in the Kabul area to base the new mission, MacKay said.

With files from The Associated Press