CALGARY, Alberta -- The head of U.S. Central Command warned Friday that the thousands of American troops surging into Afghanistan's turbulent Helmand province to battle the Taliban are in for a tough fight.

Gen. David Petraeus, best known for coordinating the troop surge in Iraq that is credited with reducing that country's violence, cautioned that the Taliban are resilient fighters.

About 68,000 American troops will be on the ground in Afghanistan by this fall  -- double previous numbers. That's compared to about 2,700 Canadian soldiers.

"I think you have to recognize this is an enemy that is adaptable and at times is barbaric," he said. "(They) adjust to our tactics, techniques and procedures. We certainly do see it as an enemy that represents an ideology that does not tolerate those who do not think the way they do."

Petraeus, who oversees U.S. military operations in a region that stretches from North Africa to the Middle East, is visiting Calgary, Alberta, at the invitation of Gen. Walter Natynczyk, Canada's chief of defense staff.

Afghanistan, where 121 Canadian soldiers have died, has been a challenge for coalition troops since the NATO mission began in 2002. Canada's mission to Afghanistan set to expire in 2011.

Although coalition forces have been able to clear insurgents from hotly contested areas, the Taliban return when foreign troops leave.

The new American offensive hopes to stop that. About 4,000 U.S. marines and 650 Afghan troops are now moving through Helmand, west of Kandahar province. U.S. forces plan to hold freed areas until Afghan security forces can gain a foothold.

Reinforcements for Kandahar will be part of the next wave of troops, Petraeus said.

"The next group that deploys is a Stryker Brigade combat team  -- it's a very substantial force," he said.

The Stryker Brigades are built around eight-wheeled armored vehicles that are very similar to the LAV 3s that are used by Canadian battle groups in Afghanistan.

"You know the Stryker vehicle has proven to be very sought after in Iraq. It has superb connectivity, satellite (communications), distributed command and controls and it is well protected for an environment that has seen increasing use of improvised explosive devices," Petraeus said.

The new troops will be led by U.S. Gen. Stanley McChrystal, who was President Barack Obama's recent choice to take over the NATO-led conflict in Afghanistan.

All commanders have received new orders to prevent troops from shooting at the Taliban if there is any risk of civilian casualties.

"We've seen violence go up in the fighting seasons in each of these last two years to levels considerably above those in previous years," Petraeus said. "So we've got to stop that trend and then we have to reverse it."